The issue is actually whether or not they tell the N900 that they're dumb sources. The USB Consortium has dictated that the way to do this is to short out the data pins (and this is what the N900 looks for) - unfortunately this has only happened in a recent standards revision, so older hardware often just leaves the data pins completely disconnected (i.e. open circuit). Presumably the change is because there's no way to differentiate between an open circuit and a smart system that's refusing to respond.