Harmattan will be on the next device. This device will be compatible with MeeGo, but it is not MeeGo. The N900 is the development platform for MeeGo. There will be (and is) MeeGo ports for the N900.
Harmattan has, at this point, nothing to do at all in any fashion way shape form or transparent mist with the N900. There is, however, a thread started by qgil about working on a Community Hacker Edition of Harmattan to run on the N900.