I think this is a decision that you really need to make after analysing your particular market. If you have a lot of government/corporate clients who may be slow to move to new technology, then possibly the extra effort to get things sorted for IE 6 may be worth it.
But if like 37Signals, your market is more made up of small/medium business and tech savvy users who move relatively quickly to new technology, then possibly the time to say goodbye to IE 6 is now.
Only you know for sure.
Companies can thrive on less than 100% of the market; look at Apple as one example. You don't have to win everyone over to succeed -- and in this case, IE6 share will continue to decrease with time, so your market segment will increase likewise.
Don't kick IE6 users out, but don't go out of your way to make the experience flawless. If the issues are fairly noticeable, you can put a note saying "You're using an outdated browser; for the best experience please upgrade."
For a classifieds site with broad appeal:
UBs: 4,828,839
IE6%: 24.82%
As a point of interest -
FF%: 17.17%
Safari: 3.97%
Chrome: 0.70%
Unless you are catering to a predominantly tech audience, you've got some pretty hard decisions to make in not explicitly supporting IE6.
If you're working towards your first release I'd be inclined to trade IE6 compatibility for velocity.
Best of luck!