That's somewhat of a hot question at the moment. There's certainly plenty of community support at the moment, and the hope is that either Maemo 6 will be available for the N900, or that the majority of applications will be compatible with both (or trivially portable). The move to Qt4.6 as a cross-platform library for M5, M6, and Symbian should help in this regard. Nokia haven't given any concrete assurances yet though, so it's all a bit up-in-the-air. Failing that, community support could drop off pretty sharply I'm optimistic on that front though.