I thought the N800 concept was a great one and I really enjoyed using mine. I didn't miss the cellular functionality it lacked. It only disappointed insofar as it felt like a concept device that Nokia pretty much abandoned to small apps developers.
I loved the fact that it ran a form of Linux and I could compile my own apps for it.
The N900 looks like an incremental development of the N800 tbh and I can't see anything revolutionary about it. I hope they have endowed it with enough cpu cycles and RAM as they were the chief things lacking in the N800 harware-wise.