On the consumer Internet sites I have worked on in the past year, the stats have shown roughly 30% of users still have IE6. For the services that 37Signals provides, it's quite likely that this drops to just a few percent -- after all, their customers are tech-savvy enough to be using Firefox, Safari, and IE7. And when you've got just a few percent of IE6 users, it just doesn't make good financial sense to spend a disproportionate amount of time dealing with IE6's inane bugs.