I spent the start of the holiday break upgrading my studio gear, adding an HP LP2475w monitor and Logitech Z-523 speakers, both of which are excellent. My Macbook now runs in clamshell mode, with a wireless keyboard and mouse.

The new studio setup in action
Games in particular are vastly improved on the new widescreen, but I had a devil of a time getting the wireless keyboard working under Boot Camp and Windows XP. Let me save you the hours (!!) I spent getting it sorted.
Note that this is for Windows XP. It might work for Windows 7—if you try it let me know what you find out. And although I haven't tried it, I think this process should work for the wireless mouse as well. I've got a Magic Mouse on order and will post an update once I've had a chance to try it out (Update: yep, it works the same, just choose "No passcode". Note that initially it said that the drivers failed to install properly, but a few seconds later the mouse kicked in and has been working fine ever since.)
The magic formula
I started by unpairing the keyboard from OS X. I don't think this is strictly necessary, but I'll mention it just in case. If you follow the steps below and it still isn't working you might want to try it out. In System Preferences > Bluetooth, select the keyboard and then press the minus button at the bottom of the list and confirm the warning dialog.
In Windows XP, bring up the Bluetooth control panel. Make sure the keyboard is turned on, then let it scan for devices. After several seconds the keyboard should appear in the list. Right-click on it and choose Properties and then Services. You'll see a checkbox options for drivers; check it and click Apply.
Now return to the device list, select the keyboard and hit Next to begin the setup. When asked for a passkey, choose Use the passkey found in the documentation and enter 0000 (that's four zeros; of course this requires that you have a keyboard connected! If you don't, use the On-Screen Keyboard application found under Accessories > Accessibility). Hit Next again and you'll be prompted to verify the passkey on the wireless keyboard. Enter the four zeros and press Enter (on the wireless keyboard). After a second it should confirm the device setup.
That's it, you're paired. Reboot XP once just to verify it will pair. Note that the special keys (brightness, eject, etc.) only work if you're running on one of the newest iMacs, or are using the 2.2 (Leopard) version of Boot Camp. Hopefully this will get fixed in a Boot Camp update, but considering how long us MacBook users have been suffering with audio issues I won't hold my breath.
Hopefully you're up and running at this point. If you unpaired the keyboard in OS X you can now reconnect it. And in case you're still stuck, here's Microsoft's troubleshooting guide.
Update: If you are having problems with your Magic Mouse "freezing" for several seconds at a time, particularly when doing CPU intensive stuff like playing games, see this thread on the Apple support site for a suggestion that might help.

Ah, the original