Maybe our definition of a MID isn't the same then.
What I mean when I say MID is a device that will run everything that runs on my desktop - as long as memory/CPU-requirement don't prevent it.

While the N900 has more horsepower and might be able to run resource-hungry applications in theory, I cannot imagine Abiword/Gnumeric/Xchat/... on a 3.5"-screen with those giant Maemo5-UI-elements. (See how the Xchat preferences screen already is too big for the N8x0. No way it would be useable without major rework on a smaller device.)

Also I expect a MID to be a perfect video playback device. On the N8x0, subtitles in DVDs are barely readable, even if the film plays fullscreen and is cropped to 15:9. I'm almost certain they're imposssible to read on the N900.

After 3 years with all internet tablets Nokia produced so far, I know they are almost perfect for my needs, only a little bit too small. Making them even smaller isn't going to help.