Nokia remotes are only partly compatible with the N900, read this as an example : https://bugs.maemo.org/show_bug.cgi?id=7945
it says the bug is fixed, but it is just not considered a bug by the Nokia bug guy. Looks like the N900 isn't supposed to support more than the "answer phone" button.

what's certain is that remotes made for Apple products are not compatible, Apple came up with a different wiring than what Nokia already had when they designed their remotes.