"official MeeGo" = the release of an open source platform by the MeeGo project at meego.com. A community effort, including (soon) the tools to make all kinds of customizations.

If you mean "What will be the difference between the MeeGo-Harmattan release from Nokia and the MeeGo-Harmattanm release from the community", the answer is: nobody knows today. At this point nobody knows what will and will not work. Technical discussion, R&D experimentation and official MeeGo-Harmattan releases need to come first in order to tell.

The main points to watch are: hardware adaptation for the N900, impact of hardware differences between the N900 and the new target device from Nokia, software performance, possibility to redistribute Nokia binaries and possibility to redistribute 3rd party binaries (being each 3rd party a different story). Probably another piece to look at is the security framework and what can or can't be done with devices that didn't have it originally installed.