Who would have thought that, years after helping start the desktop revolution, Apple Computer (NASDAQ:AAPL) wouldn't be so much about hardware and operating systems as it is about the music business? For those who can remember the days of Atari 800XLs and Amigas and Radio Shack TRS80s (a friend of mine would always call it the "Trash80"), I doubt that many would have necessarily predicted that Steve Jobs would eventually find succor in the world of pop-tune distribution.

He did, though. While Microsoft (NASDAQ:MSFT) holds the monopoly on operating systems and Dell (NASDAQ:DELL) is the royal delivery vehicle of that monopoly, Apple continues to kill even further with its own killer app: the iPod. The public is absolutely crazy over this device, crushing on it like a male teen instantly taking to the first centerfold he's ever seen. I myself have never used one, but then again, I hardly use cell phones and don't wear a watch; portable electronic technology has apparently passed me by.

And the musical juggernaut continues, for Apple recently announced the latest mark to be passed at its iTunes store: Over 200 million songs have been downloaded. Seems like only yesterday -- or, last July, to be specific -- when Tim Beyers wrote a column on iTunes passing the century mark. In that piece, Tim brought up a funny but excellent analogy: When will Apple be able to say that it has served over 1 billion iTunes, just like McDonald's? You have to wonder; doubling from summer to winter is indeed hot growth, but I'd have to assume that growth is going to slow a bit since the numbers are getting larger.

Then again, who can say? Everyone knows someone with an iPod by now; in fact, it finally happened to me, when a friend of mine recently acquired one as a gift. The devices will certainly be under enough yuletide trees this season to constitute a forest, and it's a safe bet that prepaid cards good at the iTunes store will be packaged with many of those presents; the cards can be acquired at places like Best Buy (NYSE:BBY) and Circuit City (NYSE:CC). In fact, according to a recent article in The Wall Street Journal, demand is so high this holiday season that some retail outlets have sold out and are eagerly hoping to be restocked as soon as possible to capture those iPod revenues.

As far as investors in Apple stock are concerned, this milestone is noteworthy, but many are probably still waiting for the other shoe to drop in terms of the whole iPod program -- that is, they hope the device converts many of the music lovers into owners of an Apple desktop machine. If such a conversion trend were to appear and be solidly sustainable, then the rich valuation of the stock -- which Seth Jayson appropriately has severe reservations about -- might be justifiable. Then again, the aforementioned Mr. Beyers brings up some good counterpoints to Seth's negative thesis on the other side of the duel over Apple's prospects.

The stock has been on a pretty bullish run as of late (what's the name of that song U2 sings in that commercial?); there's no denying that. But will it continue, and will Apple's fundamentals catch up with its momentum? We'll have to stay tuned to find out.

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Fool contributor Steven Mallas owns none of the companies mentioned.