Apple (Nasdaq: AAPL) started out 2011 in style by hitting a nice, arbitrary milestone: The company's market cap now stands above the $300 billion mark.

To put that figure into some kind of perspective, you have to resort to ridiculous exercises. If you could liquidate Apple today at full market value (and not counting the $25.6 billion of cash equivalents on Apple's balance sheet), you could:

  • Fund both the Apollo space program ($120 billion in inflation-adjusted dollars) and the International Space Station (estimates top out at $160 billion). You'd even have $20 billion left over to build a couple of large hadron colliders.
  • Install fiber broadband to 93% of Australian homes and businesses -- about five times over. Or, make a rather large dent in the woefully lacking broadband infrastructure of the United States, perhaps by buying into the audacious fiber networking plans Google is brewing.
  • Buy out AT&T (NYSE: T) and spend $125 billion to make its networks capable of handling iPhone and iPad-related data traffic -- and maybe even complete the occasional voice call without dropping the connection.
  • Buy a 32-gigabyte iPad with WiFi and 3G connectivity for every man, woman, and child in North America. Talk about a virtuous circle!

Apple would need another 25% boost to pass ExxonMobil (NYSE: XOM) and become the largest company in the world as measured by market cap. Apple generated $14 billion of net income over the past four quarters, which is about half of Exxon's total.

I'm not saying that Apple needs to be worth less than half of what Exxon is fetching -- after all, Apple is still a growth stock and Exxon is not. But I am questioning whether Cupertino can support this nosebleed valuation for much longer using its current playbook. Sure, the iPhone and iPad are nice, but not very different from scaled-up iPods with some extra features. At some point, the addressable market will be saturated and Apple's stock will drop back to earth.

I'm not calling Apple out as the worst stock for the year, as I did in 2010 with terrible results. It could, however, be the worst stock to own over the next 10 years as the realities of competition and Steve Jobs' handwaving magic will evaporate.

If Apple is the biggest company in the world by 2021, I'll eat my left shoe -- hold the salt.

Fool contributor Anders Bylund owns shares of Google but doesn't hold a position in any of the other companies discussed here. Google is a Motley Fool Inside Value selection and a Motley Fool Rule Breakers pick. Apple is a Motley Fool Stock Advisor recommendation. The Fool owns shares of Apple, ExxonMobil, and Google. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. You can check out Anders' holdings and a concise bio if you like, and The Motley Fool is investors writing for investors.