Ouch. You know it's a bad day for Pixelmator Team when they have to publicly announce a major bug in their marquee app. The bug causes some Macs to "restart when intensively using Pixelmator" -- like right when you're in the middle of a major project. The issue also causes other nice side effects, such as "the app would stop responding without restarting a computer, or just cause some graphical glitches."
Well, the Pixelmator Team has posted a huge apology to users on their blog, and they've found out what's happened. Apparently, the issue is caused by NVIDIA GeForce graphics card drivers and not by Pixelmator's code. The team notes that they've tried various workarounds and nothing has worked, but the hardware engineers at Apple are on the job trying to figure out a fix.
If you have a Mac with an NVIDIA GeForce GT 120, GT 320M, GT 330M, 9400M or GT 9600 graphics card and use Pixelmator, you may want to use another image editing app for a while until this gets resolved. To find out what graphics card is in your Mac, pull up OS X 10.8.2's System Information Utility and it will be listed in the overview.