The problem with making a seperate forum with a pre-n900 focus is that your not going get any of the existing developers on there unless they're sticking only to the pre-n900 tablets. There are still applications or projects that are cross-platform (e.g. flipclock, dialcentral, easy Debian)

Splitting the forums into pre-n900 and n900 won't help because then the developers are going have to tend to two seperate topics for their applications and one of them isn't going get as much focus.

An enforced tagging system (e.g. it wouldn't allow threads without the tags) would allow people to see that this application is for n800/n810 and n900. But the resulting problem is that posts in that thread could range from n800/n810 to n900 and then everyone has to sift through those.