I have a Mac Mini running OS X 10.4. I've had a Microsoft wireless keyboard and mouse combo working with this machine for a year or two. Recently the multimedia keys on the keyboard have stopped working.
I've checked the settings under "Microsoft Keyboard" in System Preferences and everything seems to be configured correctly. I can't think of anything that has changed recently that might have caused this problem (except perhaps the upgrade to iTunes 9?).
The keyboard is a "Microsoft Wireless Multimedia Keyboard 1.1" according to the sticker on the bottom.
Edit: I've plugged the keyboard into my laptop running Windows XP and the multimedia keys work fine there so the hardware is not defective.