The HiFiBerry Digi user “Pretender” has written a very good guide how to use the board to output Dolby Digital and DTS to an external home theater system. This will output the plain AC3/DTS data stream without decoding it on the Raspberry Pi. Check out his guide.
P.S. We expect that this should work out-of-the-box in Raspbmc soon.
Sam Nazarko has released a new version of the Raspbmc distribution. There are a lot of interesting changes but one is especially exciting to us: It now has ALSA sound card support including a kernel with HiFiBerry DAC support.
We did not have the time to test it yet but we will do this soon! Did somebody test it already?
Update 29.12.: It seems that one necessary kernel module for HiFiBerry is still missing in the Rasmbmc december release. We will check with the Raspbmc developer to see how this can be fixed.