I would bet you every last dollar I own that IE 6 users are, on average, the most lucrative users out there.  You know how people here say "who looks at ads?"  You can't click ads you don't see.  And remember, standard user studies say that a large segment of Google users think the ads on SERPs are actually search results.<p>In short, IE 6 users are dumber than users of other browsers and Google's revenue is juiced by dumb users who click their ads.