I just had the TrueHD revelation this week too. I had my PS3 set to Bitstream the whole time and didn't realize it had to be changed for TrueHD.
For Dolby TrueHD, it has to be set to Linear PCM (you can do this in the settings menu from the main PS3 menus, or while watching a movie, you can bring up the on-screen menu (Triangle button) and choose the AV Settings icon. From there, choose Linear PCM. At this point, if you press select, you will see that it is (should be) outputting in Dolby TrueHD.
The confusing thing is that the Denon will NOT say it is receiving TrueHD, but receiving Multichannel in. Remember that if you have a 7.1 setup, you need to set the default surround back mode in the Denon for each different type of audio input (DTS, Dolby, and Multichannel in). I went for a few days listening to TrueHD without my rear surrounds engaged at all because I didn't realize that I had to set up the surround mode separately for Multichannel in.
It is undoubtedly confusing and I'm still learning something new almost every day about either the Denon, PS3, or Axioms.
Finally, if you're looking for an amazing Blu-Ray to hear TrueHD and the Axioms at their best, get the Dave Matthews Band Live Blu-Ray. It is nothing short of amazing and I'm not even a huge DMB fan.
