Qt, of course. I did use GTK+ a while ago and I don't like it - plain C doesn't fit well for object oriented programming. Qt's C++ is much clearer. Qt is cross-platform: my application runs on MS Windows, Linux, Windows Mobile (crawls) and Maemo. I'm shure I'll port it to the MeeGo without problems.
Also you can do main job in the desktop version and make only the Maemo-specific features in scratchbox or on device. It's much simpler.