I bought the Logitech Cordless Desktop MX for Bluetooth quite some time ago. I was disappointed to find out that very few of the extra buttons worked with the Mac, and the Logitech software actually broke things. Recently, I have been watching the progress of Controller Mate, and it now supports the opening of AppleScripts and Finder items. So with just a little work (i.e. following the very basic instructions and writing a few scripts), all of the extra keys on my keyboard are now fully supported.
Everyone with an unsupported keyboard should give this piece of software a try (and no, I'm not affiliated with them).
[robg adds: I haven't tested this one, but the app is shareware and only $10 if you choose to buy, so I thought it worth a mention for those who might wish to get the extra keys on their keyboards working...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050719174516155