With the N900 Nokia officially introduced the device class of "mobile computers" (to distinguish them from mere smartphones).
Future mobile computers from Nokia will run MeeGo, which is essentially just based on Maemo 6 with the OS level stuff being based on Intel's Moblin instead of Debian. Actually, Harmattan might be a hybrid with a Debian-based OS.
The UI of the free MeeGo reference handset implementation from meego.com is certainly not what Harmattan will look like.