First of all, I like Paint.Net, I have started another thread regarding performance, and now this regarding alpha masks, but besides those two issues, thumbs up.
Is there any plan to add photoshopish alpha channel editing? I saw there was a plugin to import alpha masks, and that's good, but it is a hassle if you actually work with a texture for instance not to be able to just draw in the alpha channel and see the result right away.