Figured I would bump this since I just picked up one of the Zalman ZM-RSSC units refurbished for cheap.
For the most part, this item works with mac.
Using the S/PDIF or Optical output from the ZM-RSSC, I am only able to achieve 2.0 sound in Snow Leopard. However, if I hook up the three analog outputs, the card can send a 5.1 signal (I tested a trailer from Apple's website that was using a 5.1 AAC track) to my receiver through the analog inputs. This of course means I have to hook up three 3.5mm to stereo RCA jacks instead of just one optical cable, which I'm a little disappointed by but I can't find anything that would do otherwise. Even the famed Griffin Firewave had to hook up this was so whatever.
My main reason for getting this product is to be able to monitor 5.1 soundtracks while editing in Soundtrack Pro. If I run into further issues, I'll repost but so far the answer appears to be this- it works, but only with the analog outputs. That said, if anyone has a fix for using the optical port on this unit, I sure would appreciate it.