Dunno who told you that but it's not making any sense.
1. the maps themselves are a legacy product, meaning they are compatible on every nokia device be it symbian or maemo.
2. mapping is done by navteq that is a separate division of nokia, so the quiestion is not "if" but "when".
3. old devices like N95 and 5800 get firmware and map application upgredes so it's doubtfull N900 will be "phased out" in the next 3-4 years.

Checed OSM but the main map does not load in a backgroud thred, thus becomming choppy - have to talk to the project mantainer about that.

As for android I find the support scarce and garmin still didn't provide gps on the devices.
Garmin is the only one providing support for some parts of the world.