I doubt this will happen anytime soon. Pixelmator is heavily dependent on OSX-specific tech, like core graphics. Porting it to other platforms would require basically reimplementing all this functionality from scratch, making it performant, and then recreating the entire UI.
Indeed; ~half of the technologies listed in their splash image for the "New Image Editing Engine" are Apple-specific: vImage, Accelerate framework, vDSP, Core Animation, ColorSync, Core Image, ...
Some of these have rough equivalents on other platforms, but the interfaces definitely don't line up exactly, so it wouldn't be completely straightforward to shim them. Others simply don't have existing equivalents.
Leveraging Apple's native frameworks such as Core Image make it much, much faster to implement something like Pixelmator on OS X than any other platform. It makes it very enticing as a graphics app developer to go Mac-only and cut your workload by a factor of 10.
I recently stumbled on Chasys Draw Artist. It's pretty amazing coming from one person. Many more features over Paint.net, yet startup time is about the same as compared to Gimp.
I would also really like to see this, but it doesn't look like it'll ever happen. Linux doesn't have any good alternatives to Photoshop, GIMP just isn't there in terms of usability. I use GIMP for quick edits, cropping, etc. But I just can't use it for more than that.
Even if they don't make it cross platform, it will probably give Adobe a run for their money. Progress on this thing had been astoundingly fast, and compared to Adobe it's practically free.
