I thought I'd let everyone here know something I've noticed since I had an issue w/ some BD not playing 5.1 surround sound in case anyone else is frustrated w/ it.
The first day I had my PS3 I played "Four Brothers" on BD and everything was fine... well I went to tweaking some audio options and noticed that the audio output for both HDMI and Optical was set to bitstream. Knowing what little bit about how signal processing works, I knew that PCM would be the better choice of the two options and selected to set my audio outputs to PCM instead of bitstream.
A few days later everything seemed to be working fine (games and what not) till I played "Talladega Nights" and discovered that my surround sound processor showed that I was receiving PCM but it was only in 2 channels. I didn't have time to futz with it because I had guests over and just played it really loud in 2 channel mode. A few days later I put in "Superman" on BD and still only 2 channel mode. This time I was furious and decided to figure out what was going on. So I stopped the movie and screwed around in the menus until I decided to take it off PCM and move it back down to Bitstream. Of course it says something like you won't get full effects using bitstream but I tried it anyway and 5.1 worked fine. Apparently the PCM is set up for 7.1 channel receivers and not 5.1 receivers. However the bitstream works well.
I hope this will help someone either now or in the future from getting as frustrated as I had been and wondering if it was a PS3 issue or a setting issue.