If you mean "that" UI, MeeGo 1.0, posted on YouTube, yes, I think N900 already has it. But I guess It's not possible to say that if that UI is the same as which will be shipped with N9, since N9 is not officially announced or any screenshots/videos posted -again, officially- about it yet.

Anyway, Its highly possible to use it on N900 too, It's an open-source software -like Hildon- and If somebody puts enough effort on it, It can be ported.

As far as I remember, Hakulinen said "PR1.3 is soon" and It comes with Dualbooting to MeeGo feature, so It would be meaningless to release MeeGo for N900 without dualboot feature, I think PR1.3 will be released first, then MeeGo for N900 will follow.