It appears that the issue has now been resolved with the release of OS X 10.8.3 last week. From Pixelmator's blog:
Since the release of the OS X 10.8.3 update last Thursday, we’ve been thoroughly testing Pixelmator on it. We can finally confirm that Pixelmator’s biggest problem (caused by NVIDIA graphics card drivers) is now solved! For that, we owe a tremendous thanks to the guys at Apple and NVIDIA for listening to us and probably many other developers, and then doing something about it.
Go ahead, everyone, and download the OS X 10.8.3 update to your Macs!