In general, they do nothing. Apple laptops don't have these keys, so there can't really be any major OS X functions mapped to them. Some sophisticated pro-level apps may be able to take advantage of them, and you can also map other common functions to them in System Preferences --> Keyboard & Mouse --> Keyboard Shortcuts.
F14 and F15 should adjust the brightness of your display.
You can assign them to in System Preference - Keyboard- Keyboard Shortcuts(or something like that, currently away from my mac)
Or if you want to be able to assign them to change tracks in iTunes etc. then you can use an application like sizzlingkeys. May also want to check out onekey or something like that enables you to assign more complicated things to it, like applescripts etc.
Solution: Disable shortcuts under "Display" in system Preferences, Keyboard Shortcuts
Just for the record.... The solution was to go into the Keyboard Shortcuts in the System Preferences and, under Display, uncheck the entries for increasing/decreasing brightness.
I found that in the QuicKeys FAQ. I'm not actually using QuicKeys, but I'm pretty sure it's the same problem. Until you do what I've described here, it seems that nothing else in the system can detect that F14 and F15 have been pressed. There's also some great info on Mac keyboard remapping at Jacob Rus' page on the Cocoa Text System.
I know this is an old post, but I wasted a bunch of time trying to solve this tonight, finding a lot of forum threads like this with the same question, but but no satisfactory answer.