http://www.neogaf.com/forum/showpost.php?p=90673295&postcount=240
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. Even if you have a Dolby only HDMI receiver (which I'm not sure exists), you will still get 5.1 or 7.1 sound since those receivers should accept uncompressed surround.
For the Dolby only headsets, my understanding is that these will work but you will only get stereo audio since we only pass Stereo and DTS through the optical port. I have not tested this myself, but I'm told it works. Regardless, I understand this is an inconvenience, but again we're going to have Dolby coming.
This is unrelated to the HDMI-IN "Surround Sound" beta. To clarify, we default HDMI-IN audio to be converted to Stereo. However, we do have a feature you can access in TV settings/Troubleshooting that enables Surround Sound in "beta" form. If you check the box, and you get Surround, you're golden. We found some inconsistencies in STB's during testing and decided to disable it by default to insure a good initial setup experience for people.