Someone just wrote an interesting app for Maemo (Nokia's linux for its N810 "internet tablet") called BlueMaemo
I'd modify it as follows:
1) doesn't currently work with Mac OS X -- add that.
2) add support for remote devices that support Bluetooth BIP (specifically, the ability of a BIP device to remotely display its screen).
Now you can remotely display another device's screen on your Android device, and use your keyboard to type on it, and use your screen as a trackpad for it.
And, one more: make it so that Android can support Bluetooth HID and BIP as well, so that you can port that app to run on Windows/Mac/Linux, and remotely control your Android phone from your desktop or laptop computer.