If you have Pixelmator 1.6.3 installed on your Mac, don't be fooled if running "Check Now" in Preferences => Updates tells you that you are running the latest version. 1.6.4 is waiting for you on the Mac App Store!
What? Pay for it again? Well, as an email sent to licensed users says "You can download Pixelmator on the Mac App Store for just $29, for a limited time. By transitioning to the Mac App Store, you will get the totally awesome Pixelmator 2.0 (and, of course, still lots of 1.X updates) for free once it is out in the Mac App Store later this year."
So think of it as a prepaid upgrade. True, those who hadn't yet paid for the program will get the same deal, but at least they'll be joining you in helping to underwrite one of the finer pieces of software in existence.
If you wait, the price will go up, so act now.