From an operational perspective if they feel that this won't have too extreme an impact on their customers I think it makes a ton of sense from a productivity standpoint.
So do you think it's better for most users if they can fix bugs faster, respond to support requests faster, build new products, add new features (<gasp>) etc? I think it is. And I think blowing off IE6 will allow them to server other folks better.
Time is always allocated somewhere-- the time they save by giving up IE6 support can go to more constructive/forward thinking pursuits.
Only 37s has access to their web stats-- if ie6 is vanishingly small within their paying userbase, it's a smart move.
No one complains about IE5.5 not being supported by most sites anymore, but people still use it.
37signals has always extolled the mantra of usability. But, this move puts them entirely against that. You need to work within the constraints of the user, and one of them, unfortunately, is IE6.