Regarding the "hidden" DD 5.1 track in TrueHD.. that track only exists in the BRD specs, not the HDDVD specs. Thats why HDDVD requires a DD+ track to be included on discs. (High Def Digest put together an article that explains this difference between the two specs).. notwithstanding I Am Legend where WB decided they didnt need to roll with a DD+ track.
so, if it is playing TrueHD in 5.1 off an HDDVD the player is decoding the lossless TrueHD track. Then they output that as PCM (lossless). If you have optical and dont have the bandwidth for PCM 5.1 it will be re-encoded into DTS like the Toshiba HDDVD players did.
So the only question left, are we getting the DTS-MA track or the core? Its in both HDDVD and BRD specs to have a core "DTS" track, so without Sammy coming out and commenting on it, we will never know.