There are no official(as in end user support by Nokia) MeeGos for N900, but there are two community projects:

1. MeeGo proper (MeeGo core + MeeGo reference UX), this is done under the MeeGo project with the N900 as an official "reference device" with a professional team doing the porting.

2. MeeGo-Harmattan HE (Maemo 6 core + Nokia MeeGo UX), this is to bring the software on the N900 successor to the N900, including the proprietary bits. Work on this hasn't started yet as there's not even a platform SDK out. Can possibly benefit from the hardware adaptation work done for 1)

That's how I see it.. again, someone please correct if I'm wrong.

On-topic:

If MeeGo-Harmattan HE works out, applications for the N900 successor should work. What AFAIK no-one in the know still hasn't elaborated on is the implications of the security framework, open and closed modes etc. This might possibly have effects regarding Ovi Store and commercial software for example.