[EDIT] The "?/Help" screen in Roku Media Player lists the supported codecs; presumably this list is customized to the actual capabilities of each model? For me, it lists AAC but not AC3, and then it further states that Dolby Digital is passed through from MKV, MP4, and MOV files. So I guess that answers my question about the Roku. Still leaves the question of why my tv isn't able to decode AC3 (the audio standard for broadcast digital tv) over HDMI.....
What is your settings for digital audio? It usually should be left at Auto, and the Audio Mode should also be Auto.
When digital audio is set to Auto, it should tell you what audio codecs the TV is telling the Roku that it can decode.