A major challenge of web development is that a particular OS and browser of the end user is not guaranteed, and so many must be catered for. This is not as bad as it used to be, now that Netscape 4, and Internet Explorer 5 and earlier have been consigned to history. This leave IE6, 7 and soon 8, Firefox, Safari and Opera. That is still plenty of scope for variation, and the general problem is IE6. So the decision by 37Signals to discontinue support for IE6 in mid August is an interesting development. It is also a bold move, but that is the way 37 Signals so things, by not catering to the entire world.
However, does the rest of the web design community have the same luxury? For all the sites I manage, IE 6 is still the 2nd or 3rd most used browser. It depends on the actual content and target audience, but overall IE6 still has a usage range between 10 and 30%. That is a large group of people, that no client I have met is going to want to even contemplate missing out on.
The work arounds for IE6 are well known, and its conditional stylesheets and alike are going to be around for a while yet in my web development.