I wouldn't worry that everybody suddenly abandons the N8x0.

Going from N8x0 to N900 is imo a completely different thing than going from N700 to N8x0.

Compared to it's predecessors the N900 is covering different usecases. While the N900 certainly brings some nice improvements, there are also some disadvantages, where Nokia had to make compromises due to the smaller form factor. Smaller screen, less hardware keys, probably reduced battery life...
And it's much more expensive than the previous internet tablets.

So I guess quite a few people will stick to their N8x0 ITs and wait at least for the next Maemo device after the N900. Especially the ones who don't really need a smartphone, but just want an internet tablet.