Just out of curiosity, I want to ask xxxxts what it is more precisely with the n900 that doesn't cut it anymore. Ok, I understand the specs alright, well, at least the cpu is getting a little outdated (think the gpu part is still pretty strong though!).
But if you are the person that doesn't care whether the phone (urrhm, tablet) is somewhat thicker I can't see what it is that you're missing in the n900.
I will stick to my n900 because there is nothing out there that has a linux based OS that is as close to the real thing as the n900. The ssh client and ssh daemon are full featured and the shell feels like the real thing with python, perl, debian packaging and all that good stuff.

For me I gotta say, I have just bought the asus transformer prime tablet, its a droid and after a couple of weeks I have to say that in no way near android feels as good as the n900. Its not real linux I gotta tell you, its just the kernel in the bottom, all else above feels completely alien to me.
I have seen a few physical keyboards also, and they all feel like crap in comparison to the n900.
Look at all the other nice plus'es to brag with that you've got;
Stereo speakers, a table-stand, customizable status led, integrated skype (all other skype "apps" I've seen is a joke).
You got REAL multitasking. You got Qt for development.
Up to 10 desktops. Widgets who are so customizable that they can look like just almost anything (not like the blocky boring iCrap icons).

What the hell? What more could anyone want. There's just nothing out there that cuts it if you take the n900 as a baseline.