you can run pretty much any onboard codec in surround either via digital or analog. There's a feature in the preference pane to combine multiple jacks for surround output.
If you want an external 5.1 solution, look for Mac-only (or Mac-centric) devices like the Griffin Firewave.
Also, real Macs support 5.1 via a TOSLink cable, so it's not going to be a limitation of the OS, only of your hardware and the accompanying drivers.
I do not understand what I do. For what I want to do I do not do, but what I hate I do.