A comment on konttori's blog raised a good question for me, as I cant stand white colored electronics, could a ps3 controller be used?
It's a little more complex than you would hope.
First, bluez needs a patch applied and a few apps need to be compiled. Then, you need to physically connect the PS3 controller to the N900 via USB. The PS3 controllers require it because they only pair over USB, not by Bluetooth alone.
Once paired, you can use it fully wirelessly over Bluetooth. Unfortunately, I haven't done all this myself as I don't have a Micro to Mini USB cable around.
So, presumably this would USB host mode on the N900?
I was just thinking that that would be required and therefore wouldn't work.
You may be able to get it to work by pairing it with a desktop machine that is spoofing it bdaddr to match the N900's and then copying over the negotiated link-key to the n900.