I've yet to see any other "smartphone" do what the n900 can potentially do with running things like KDE and Debian (based off of what was possible with the n800/n810). Well besides the G1 which can also run Debian and OpenOffice.
Edit: I guess an easy way to phrase it would be: Would adding cellular voice capability to any device that is decently portable . . . make it a smartphone or a computer? . . . That's how I see the n900, just the new internet tablet with cellular capability (more so for the mobile data rather than voice). Though understandably not everyone thinks the same way.