I just visited the online shop of several national Nokia websites. In shops showing a "top sellers" list, N900 is among the best selling models in many countries.

These are my findings:

Country - Place
Italy - 1st
UK - 1st
France - 1st
Netherlands - 1st
Poland - 1st
Sweden - 1st
USA - 3rd
Germany - not in the top 3
Spain - not in the top 5