There will not be an official MeeGo-Harmattan release for the N900, but there is a possibility that the Nokia official UX and apps for MeeGo-Harmattan are made available to N900 users through a community effort discussed in (this thread). At this point nobody knows what and will not work. Technical discussion, R&D experimentation and official MeeGo-Harmattan releases need to come first in order to tell.