These days it's hard to find a pundit willing to question Apple Computer's long-term prospects or the calls of its famous CEO, Steve Jobs. After all, Apple's fortunes have been on the rise for nearly a half-decade now, and they seem to be only gaining steam.

That has caused even some of the most devoted skeptics of years past to stop fretting over Apple's future. For years, many felt that Apple's past mistakes were bound to come back to haunt the Cupertino (Calif.) company -- the refusal to license the Mac OS in the 1980s; the stale products, bloated expenses, and management turmoil that hobbled it in the mid-1990s; the software availability and falling market share that plagued it right into the 21st century.

These days, with Apple's stock price the talk of Wall St. and its products once again defining techno-chic, all that's a distant memory.

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That is, unless you're Clayton M. Christensen, the Harvard professor and author of the seminal 1997 book The Innovator's Dilemma. Christensen, who more recently wrote Seeing What's Next: Using Theories of Innovation to Predict Industry Change, isn't willing to jump on the Apple bandwagon just yet.

As well as Jobs & Co. is performing now, Christensen fears that success is built on a strategy that won't stand the test of time. Christensen spoke with BusinessWeek Computer Editor Peter Burrows on Jan. 5. Edited excerpts follow:

Apple is doing phenomenally well these days. It seems it's doing a textbook job of maintaining huge market share in digital music players, long after most experts thought that share would erode. And it's doing so with the same proprietary strategy that many thought would never stand up to an onslaught from the likes of Microsoft, and Yahoo! Can Apple keep it up?

I don't think so. Look at any industry -- not just computers and MP3 players. You also see it in aircrafts and software, and medical devices, and over and over. During the early stages of an industry, when the functionality and reliability of a product isn't yet adequate to meet customer's needs, a proprietary solution is almost always the right solution -- because it allows you to knit all the pieces together in an optimized way.

But once the technology matures and becomes good enough, industry standards emerge. That leads to the standardization of interfaces, which lets companies specialize on pieces of the overall system, and the product becomes modular.

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At that point, the competitive advantage of the early leader dissipates, and the ability to make money migrates to whoever controls the performance-defining subsystem.

In the modular PC world, that meant Microsoft and Intel, and the same thing will happen in the iPod world as well. Apple may think the proprietary iPod is their competitive advantage, but it's temporary. In the future, what will matter will be the software inside that lets users find exactly the kind of music they want to listen to, when and where they want to, with minimal effort.

But Apple has that software. It can be the one to provide that to everyone else, if it chose to, right?

I'm concerned that they'll miss it. It's the fork in the road -- and it's comparable to the fork they faced when they chose not to open up the Mac in the 1980s, when they let Microsoft become Microsoft.

How long will Apple have to make this change?

I'd be very surprised if three years from now, the proprietary architecture is as dominant as it is now. Think about the PC. Apple dominated the market in 1983, but by 1987, the industry-standard companies, such as IBM and Compaq, had begun to take over.

But let's assume Apple has learned its lesson, and that it's intent to not repeat history.

The trick is to manage the transition. [As standards take over], the products always become much lower in cost and much more broadly available [from more suppliers]. So if you're the incumbent, it appears you're facing a huge threat, even though you're really at the cusp of a great new opportunity. But it's usually new companies that grab that opportunity.

So it really is a fork in the road for Apple. If they don't open up the architecture and begin trying to be the iTunes inside all MP3 players, they're going to have to keep coming up with the next cool thing.

What about in the PC business today? Apple has been gaining share for the first time in years, and most people think that will continue, given the delay of Microsoft's Vista software, widespread malware problems with Windows, and Apple's move to the Intel platform. Don't you think that will enable Apple to gain some significant share in PCs?

I don't. I think it will allow them to survive for a bit longer. I think most people are satisfied with their current PCs (using Windows and based on Intel chips) and find that the performance of their systems is good enough. Sure, there are people at the bleeding edge who want to do more. But a good Dell PC can be had for $500, and it has performance that's well beyond what most of us need.

Seems to me, given your comments, that Apple has another strategic option: to focus on continuing to develop new markets with its proprietary, innovation-heavy approach, harvest them, and move on.

We have a case about this at Harvard [Business School], about when John Sculley was the CEO of Apple in the early 1990s. He actually had remarkably clear vision about where the industry was heading. He had three priorities.

First, he felt the company needed to get its price down to $1,000, from $3,000 or $4,000 at the time. The second thing was to open up the architecture, by selling the OS. And the third was that handheld devices were going to be big. He was right on all three, but the culture of Apple was just so strong that Sculley just couldn't change the direction of the ship.

So I always ask the students, "What would you do if you were on Apple's board?" And they always say the same thing: "Crucify him, and bring in a good manager."

"So who would you bring in?" I ask. And they say: "Bring in someone really strong, who can make those decisions." So what did Apple do? They brought in Michael Spindler -- a strong general manager type who was known for his operations ability. Well, that didn't work out.

So I ask, "What would you do next?" And they say: "Bring in a good manager -- someone who can turn the company around." Well, they brought in Gil Amelio, who had turned around National Semiconductor. But he only lasted 18 months or so.

So then they bring Jobs back. And why did the company prosper under Jobs? The students' instinct is to say, because he's a good manager. I think the reason is that he stopped trying to change the company. He wanted them to do what they had always wanted to do: make cool products, based on proprietary architectures.

One last question. It's clear that for Apple, and for Jobs, the product comes first. Rather than try to enter many new markets to achieve the revenue growth Wall Street expects, maybe Apple should just stay true to its focused approach -- in essence, tell shareholders that it's not going to try to achieve more than it can, stock price be damned.

I've been thinking about this a lot -- about whether managers ought even to think about what Wall Street says. In the 1960s, the average investor held shares for over six years.

In that world, it made sense to frame the job of the manager as maximizing shareholder value. But today, 10% of all shares are owned by hedge funds, and do you know what their average holding period is? It's just 60 days! And another 85% of the equities are owned by mutual funds and pension funds, and the average tenure there is 10 months.

Their time horizon is shorter than even that of even the shortest-term managers. So I don't think it's right to think of [these investors] as shareholders of your company. They're investors who temporarily own securities in your company at a particular point in time. They're responsible for maximizing the stock value of their investments.

You as the CEO are responsible for maximizing the long-term health of your company.

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