Careful...
I have both the Apple BT Mouse (non-Mighty) and the Apple BT Keyboard for my iBook G4 and I love them, no complaints. However -- I seem to recall a post about a year ago, where somebody's iMac BT didn't work properly, so he was screwed. The mouse or keyboard wouldn't work at all. With a laptop, however, you've always got the built-in fallback that you don't have with the iMac.
If I was buying an iMac today I'd get the wired version, as much as I like BT, simply because I like the idea of the additional USB plugs on the keyboard.