Well, for my PB, I have those Creative 6.1 speakers connected to the M-Audio Sonica using the analog outs. For the G4, the digital out is connected to a Pioneer reciever. Anyway, if you can't find M-Audio (they are a pretty big brand, not as big as Creative, but big none-the-less, and they were just bought out by AVID a few weeks ago), then I would suggest going to a store that has a good return policy and pick up the Creative product. If it worksgreat. If notjust return it.
It shouldn't be that expensive to begin with, and if you lose a few bucks even for a small restocking fee (hopefully find a store that has no restocking fee if possible), at least it wouldn't be that bad. In the past, I have even been able to convince a store manager at a place that had a restocking fee, that I wanted to buy this product that wasn't listed as mac compatible, and if I tried it and it didn't work, would he wave the fee, and he agreed.
As far as the drivers, I do believe Apple has written in rudimentary support for some Creative USB devices, mainly because the devices themselves use a standard USB audio profile and board. Any board that conforms to these specifications should theoretically work natively under OS X. Its whether or not you want plain output, or specific control over the output as to whether manufacturer drivers are needed or not.