The bug for mouse not working in Maemo was marked "Fixed in Fremantle" so in theory it should work. BT keyboards should work also, although you'll supposedly need to edit the config file to get keyboards working (and mice, too, I presume).
EDIT: My connection to t.m.o. timed out and I had to repost... In the meantime, the most excellent fanoush beat me to it with a better answer...
Tomorrow I'll leave to the summit in Amsterdam and can bring a mini bluetooth / mouse combination keyboard to test keyboard and mouse pairing and working with a N900.
If anyone have a N900 and is interested in trying - send me a private mail and we can arrange a meeting at the summit days!
In some ways its a shame the N900 does not have VGA or HDMI. Would be pretty amazing to suddenly plug it into a monitor and bang, a pocket Linux machine.
Sure you can do it over composite, but is going to look pretty bad from the distance you normally use a computer from the screen.