As far as I know the surround output through HDMI is pcm. Here's a quote from the MS guy:
Penello posted in the thread to cool matters. He said, “Dolby Digital is coming post launch. This was a SW scheduling issue pure and simple, and I know people are disappointed, but we will have it. Anyone with an HDMI receiver should be fine, as we pass the uncompressed 5.1 and 7.1 through HDMI as well as DTS.
I was under the impression that Microsoft included a workaround to output digital surround via HDMI with some super hidden BETA option... Source