At any rate, in the last couple of months while I have been using my N900 I have been fielding similar questions. The only difference is that instead of hearing "is that OSX?" (the N900 can run OSX by the way...) I hear "Hey is that one of those new android phones?" As I am working towards becoming a teacher I figure educating the masses is one of my duties in life - as such typically I inform the person asking me that no, it is no Android but a different from of mobile Linux (which typically opens the "what is Linux?" can of worms).
All in all I really cannot fault people for being misinformed but I can do my best to remedy the issue when I come across it. Any other N900 owners out there that experience this or is it just myself?