I get the impression that the underlying OS version will not actually matter that much. Maemo 6 looked to mainly be a progression of Maemo 5, so I'd assume that if you are using the same libraries for your application, it's likely it will work on both Maemo 5 and 6/MeeGo.

They said the target platforms for MeeGo are ARM and x86/Atom. So it's entirely possible that any ARM binaries/applications will work on on both Maemo 5 and MeeGo if they are written for a common toolkit (QT).

Look at most Linux distributions, Ubuntu 8 applications don't just stop working because Ubuntu 9 is out, aslong as the userland toolkits (gtk/qt) are up to date it should 'just work'.

The problems will come with updated hardware features that should be taken care of by the app developer. If its a bonus feature, use it or enable reduced functionality if its not there.

I'd be very pleased if they push MeeGo to the N900 but if not, I don't think its much to panic about.