In hindsight, now that I have an N900 in hand...

I still find the screen too small for a tablet. I'm also still holding out hope for another "slate" form factor device a la N800. I find myself rarely opening the keyboard on this one.

That said, I'm impressed with how the UI makes use of the 3.5" screen. Overall it works, and well. So it's certainly suitable, IMO for a pocketable phone-- especially one shoehorned onto a mobile computer.