just discovered Paint.net and I have to say I'm impressed how mature it is.
I have one minor suggestion:
I am (and probably lots of other people are) used to specifying resolution in pixelx/inch (pixels/cm somehow feels akward to me) .
In Paint.net's resize dialog the print size is tied to the type of unit you use for the image resolution.
i.e. if I use pixels/inch i have to specify the print size in inches, which really is a pain for me (being German and not used to inches at all)
My suggestion would be to allow for seperate units for both fields. The same also applies to the new image dialog and other dialogs that have size entry fields (canvas size).