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If you don't want to do all the work porting across the different Linux/UNIX/BSD flavors, you might want to look in to wxWidgets, if I remember correctly it just tries to wrap native widgets in to a slightly more strict framework so it can work on most systems. I've used it in the past for making things just work across platforms. Because of that you can't do much that is fancy, but it looks like you don't want to anyway being that you want something that looks native rather than shiny.
But "porting" is what I want to do!
I have built the whole project to have the whole interface easily replaced just so I can create one for each environment.