Google Must be ... Rationalized!

If I had any guts I'd short Google. They're nothing but a fast growing advertising company. 150 BILLION Market cap. Foo.
Though.... Omnicom Group (OMC) is the world's largest advertising/media company, so far as I can figure, and they only have about $10B in revenues (google is already at 9.3B). And Google has 3x their profit margin and 6x their growth (can't continue!)
But Google only has about 9x the market cap of OMC. In some sense, that makes Google cheap...
But how much can they increase sales?
US internet growth is slow and that's most of Google's revenue. They must be near ad saturation, at least in the US.
They're talking about buying billboards for pete's sake.
Ok. I don't have the guts to short them.
Maybe the Google-price will just keep going up.
That wouldn't be irrationally exhuberant
Though, as one famous economist said "If it is impossible for a trend to continue forever, it won't."
Some numbers:
Total US "Traditional" advertising 2006: $12 Billion
Estimated US Online and new media advertising: $7.6 Billion
google revenues advertising 2006 short