I think there's pretty good reason to think n900 won't run maemo 6. But despite past experience, I think nokia'll sustain support for n900 for a good few years because it's picked up a larger consumer base than they expected. Added to which, the expected hardware changes for maemo 6 mean some people will actively choose to stay behind.

And in terms of apps, people are being steered towards writing in Qt, which will run equally on both platforms (and hopefully symbian, too).