It doesn't matter how I pronounce it. I only have seen 2 N900s with my eyes. The other one is with my oldest brother, who lives about a thousand miles away. We rarely speak on the phone, and when we do we rarely speak the word "Maemo" and when we do we say "Mee-Mo" to rhyme with "me" and "mo".
Everyone else with whom I have shared this phone's awesomeness has simple referred to it as my phone or, in the case of one friend, "Your cool Linux phone."