Free premium maps content on high end phones, smart move Nokia! I predicted this will happen sooner or later when Google announced free voice navigation for Android (in the end Nokia has better navigation data available through NavTeq than Google ATM).

I think the main reason that no newer version of Ovi Maps is available for the N900 is that they are re-writing it using Qt, so they can use the same code for Symbian and Maemo in the future. They probably started writing Ovi Maps for Maemo 5 a while ago (~1 year or so) using GTK and then realized that they will be switching to Qt for Maemo 6, so they didn't put much effort in it and realized some kind of preliminary version when the N900 came out.

Given that Qt 4.6 already runs on Maemo 5 I would be very surprised if they didn't release "Ovi Maps Qt" for Maemo 5 when it is ready.

All this is pure speculation, but it does make a lot of sense in my opinion.