This used to be an easy question to answer!
Okay, which PS3 do you have? If it's one of the older "fat" models, then assuming you have the PS3 set to Linear PCM, what you are seeing is totally correct as the older "fat" PS3 cannot bitstream the Dolby TrueHD signal. Instead it decodes it onboard to Multichannel PCM. When it arrives at the amp therefore, it has no idea that it used to be encoded in the Dolby TrueHD format, all it knows is it's Multichannel PCM and therefore that's what it puts on its display. This is not a bad thing and you are still getting HD audio. So "Hooray" etc.!
If on the other hand you have the new slim PS3, you have the option of doing as above, or bitstreaming the audio to the 606 so it can decode. If you do this, then you would see Dolby TrueHD on the amp...