I'm afraid that BT Keyboards are not currently / officially supported with the N900. I know what you are going to say, 'why not'?

Well it hasn't been deemed essential by Nokia as yet but some guys have managed to get some success although not always 100% of the time.

We are trying to do something about it by registering our support for what we see as a bug. You can read more about it here:
http://talk.maemo.org/showthread.php...ight=bluetooth

Please register and vote with the instructions that Oli has provided as well. The more people request this then the more chance that something will be done about it.