FWIW I had copied my entire cities folder from my E71 where I had stored all the maps of the whole world, and it works "fine"* on the N900.

* The problem is my current location shown on the map is incorrect (offset by a few hundred metres or so). I renamed the cities folder and let Ovi Maps create a new one but the offset problem is still there. BTW Ovi Maps does create the citiies folder if it doesn't exist (and the rest of the folder structure inside).