The problem here lies in 2006. I have defended the bulk of Google's share price gains up to this point, but that's going to get a lot trickier in the near term.

Is there another $125 billion company that relies on online advertising for 99% of its revenues?

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The problem is that rivals are getting smarter. Yahoo! (Nasdaq: YHOO) launched a competitor to Google's brilliant AdSense that may make some third-party publishers pause before picking Google as their ad purveyor.

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The growing paid-search market, along with smarter competition, can play itself out in several ways. Will fraudulent clicks become a bigger issue? Will a sea of worthy alternatives make sponsors bid smarter (and lower)?

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Google's share price is built on the foregone conclusion that the company will trounce estimates, as it has in just about every quarter since going public.

I think Google will make out just fine in the long term, but I see streaks of mortality creeping in as 2006 unfolds.