Google releases free tool to migrate from Microsoft Exchange to Google Apps
It works with both Exchange 2003 and 2007, and you need to be a Google Apps Premier or Education Edition customer, but other than that... it's free, quick and seemingly painless -- employees can even use Exchange during the migration. Incidentally, if you're thinking about 'Going Google', it'll cost you $50/year per user (details here)... 'and it's never been easier!'
I can't help but think Google sat on this one until after the MIX keynote speeches. Outlook 2010 and Office's comprehensive set of web apps are just around the corner -- I wonder if Google build up enough steam before then to put a spanner in Microsoft's works.