The ATI sound implementation is not GPGPU code. It is dedicated silicon, probably brought on by the Vista DRM infection and MS twisting arms to force it on people.
In any case, R600 will be compliant with the Vista requirements and can send sound directly over a HDCP/HDMI link. We are told this is a full HD sound setup, not a cheesy 2.1 channel thing.
In contrast, NV G8x parts can't do this. They have to run an external cable from the sound chip to the GPU. This may not sound like much but it blows out several kinds of auto configuration and worse yet violates Vista logo requirements. One has to wonder if this is why NV can't seem to make a functional Vista driver six months in.
The problem with Vista is that the DRM infection mandates that you do not share S/PDIF output over unencrypted links. R600 does this by combining audio and video streams, then pumping them out over HDCP infected links. This is user antagonistic DRM, but it complies with MS logo requirements, and they don't care about user experiences any more than the content mafiaa.
Add in that the R600 can do dual-link HDCP and you are going to be swimming in bandwidth, more than enough to pipe sound down.
Nvidia's G8x on the other hand can't do dual link HDCP at all, so if you have a 30-inch monitor, you will get a black screen. At that point, sound is the least of your problems.
Basically it looks like the sound card in R600 is going to be the killer app for home theatre type apps. G8x simply can not do what is needed here, buggy drivers or not. While the DRM infection stinks, at least R600 will be able to comply. µ