Not sure which HDDVD player you've got there buddy. If it's the HDE1 then you'll not have the option available that you can''t seem to find. I've just bought a 605 also, and I have an HDE1 and a PS3. My understanding is, that the E1 cannot send raw bitstream unencoded data, except for DD and DTS. For the Hi Def audio formats it encodes everything itself and sends it via pcm to your amp. So like me an Onkyo 605 wasn't really needed because everything is done by the HDDVD player, that's why no Tru HD light comes on in the amp as the amp is just receiving the signal and sending it to the speakers.
The amp shows up as multi ch 5.1 or some such thing. But if you press the display button on the DVD player it should tell you what sound format you are currently playing. Remember in the DVD options if a Dolby Tru HD soundtrack is available it sometimes isn't the default soundtrack so it needs to be selected.
I've heard that this is not a problem your ears are still getting top quality lossless sound, its just that its being encoded by the dvd player instead of the amp. It does sound glorious by the way. Please correct me anyone if this is all baloney!!
Dee