and just to clarify...there is no hardware accellerated sound on the Mac.
You will only get 5.1 sound if you're listening to a source that has a 5.1 track encoded in a format that an external decoder can understand (say for example Dolby Digital or DTS.) There is no mac equivalent of EAX or anything like that.
Also, if you have a 6.1 or 7.1 source, and a player that can send it uninhibited to your surround decoder, then you can do 6.1 or 7.1 sound just as easily as 5.1.
you cannot, however, play any games with 3D sound, and you'll find that none exist on the mac. It might be possible to install a PC-based sound card and then install windows and play windows-based games with surround sound through that card...but the primary application for surround sound on a Mac is creation of surround sound through the use of professional audio equipment. Ironic that it's got a built-in optical audio output but no real audio hardware processing. There's not even really any software audio processing, which is actually a blessing in disguise for some people in the Home Theater enthusiast market. You can hook up any mac with an optical audio output (mac pro, G5 tower, mac mini, macbook pro, maybe some others) to an external preamp or receiver. If you import audio from, say, a DTS-encoded CD
http://www.dtsonline.com/entertainment/catalogue/music.php, you can listen to 5.1 or 6.1 music just like you listen to stereo music...with iTunes! (Must import with Apple Lossless enabled, though!)
It takes a lot of work to do the same with a PC because Windows does process all audio that goes through. Even Vista is apparently going to do this (although it will do it better, that doesn't matter if you don't want it processed at all). To top it off, only certain PC sound cards are capable in the first place, and it's decidedly "hacky" to get it working.
If you do this, be warned that you need to turn off system sounds while playing DTS-encoded music, as any system sound playing will be mixed in with the DTS and turn it back into (potentially very loud) white noise!
Anyway, that's more than you asked, but I hope it helps clear things up some.