After doing some research on the forum I found out that the following USB audio devices which include 7.1 support seem to be a good choice for OS X. As I am getting a MacBook Pro I would like to know, which of these cards is the best choice in terms of functionality and drivers.
So here are the candidates:

1. Creative Audigy 2 NX
2. M-Audio Sonica Theater USB (which seems to be hard to get)
3. Roland Edirol UA-1EX
4. Turtle Beach Audio Advantage USB

As I understood all of these cards seem to work under OS X, although I don't know if there are any disadvantages in terms of driver support, crippled functionality (due to lack of drivers) or similar.

Maybe there are other USB audio devices which can be recommended. I am not a professional (musician or something), so it just has to be suitable for 7.1 (entertainment) and offer very good Mac OS X compatibility/driver support. As I am not looking for a professional solution, it shouldn't be too pricy, too.

I bought an AOC USB dongle for $12 at MicroCenter. Worked like a charm for me. Item specifically says support for Windows 98/ME/2000/XP as well as Mac OS X 9.0/10.1. Should work with for you with no issues.

OK, but does it also feature 7.1 sound?
I had a short look on the AOC homepage, but only found a 5.1 USB audio device.

I have tried Turtle Beach Audio Advantage USB, and it is recognized by OS X as "USB Audio". Analog and digital out work. No DD or DTS passthrough. There are no Turtle Beach drivers for OS X.