Actually the N900 will support vanilla Meego. But it won't have any of the stuff you see in Harmattan-Meego. It's up to the community to backport stuff from that version of Meego to the N900's vanilla Meego.

And as long as the developers use QT 4.7 their stuff would be backwards compatible with MAEMO anyway. I'm assuming QT will continually to be upgraded in vanilla Meego.