Many sources [1] show that IE6 still has more global usage than IE7. Though I'm sure 37signals' userbase is not represented well by global data, turning away users of perhaps the most popular version of the most popular browser is an interesting move.
What is really amusing about those stats is the 77 visitors still using IE 3. I don't even have a 12 year old computer that still functions, let alone a 12 year old browser.
[1] http://en.wikipedia.org/wiki/Usage_share_of_web_browsers