Just tried this right now, and sure enough, my Onkyo is receiving a Dolby Digital signal from Half Life 2! I'm using an Asus A7N8X Deluxe v2 with Nvidia drivers.
The odd part is it appears that nothing is coming out of the lfe channel according to Nvidia's mixer panel. But its definitely sending different amounts of audio to each of the other channels separately, even though the rears appear to be just slight clones of the left and rights. Quite honestly, I have my subchannel all the way down due to neighbors, so its not big deal anyway.
I'll do some more tests - curious if things like voice is coming through center only, helicopter fly-bys are discrete, etc.
I wish more games paid attention to passing Dolby Digital through spdif as more and more of us hook up our computers to our home theaters. Doom3 for instance was horribly dissapointing cause I had to rerun three 1/8" to RCA stereo cables to get surround. Blah, no thanks.