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.
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.
I know that IE users are more likely to click on ads, but I haven't heard much about which versions are more likely to click through. Most of the visitors I get to my sites via adwords are on IE 7 and 8; but that's just anecdotal evidence.
So, if you've got something that says IE 6 users are more likely to click on advertisements than any other version of IE, I'd love to see it - maybe we should still be paying attention to them.
As much as a part of me would like to see that happen, that'd really be an odd precedent -- not just not going out of their way to continue making something work on an older buggy browser but actively blocking it from working when, presumably, it otherwise would. I'd be happy for them to pull any extra hacks/workarounds though and make the IE6 experience worse.
I think the implication is that if a user is trying to use Google Docs or other "advanced" features, then either they themselves or their support staff are likely to be willing and able to upgrade their IE client.
Search is more lowest common denominator, hence it's ongoing compatibility.
