Like another gentleman said, you need to make sure your amp is set up properly and recognizes all 7.1 speakers, then when you go into audio settings on your PS3, click the "automatic" option. The HDMI standard that all these devices use (few exceptions perhaps), provide two way communication between devices so they can recognize each other and whether they are compliant. What also happens is, much like a handshake between other commonly networked devices (i.e. pc's, phones, routers, etc.), is they share pertinent info needed to work together as well as possible. So, when you choose the "automatic" option, your ps3 will ask the connected device it's state and settings and the ps3 will set itself to output what will give the connection the most functionality. This is what you should do. Now, you WILL be stuck with the format that your source material provides, which is most commonly 2ch or 5.1. HOWEVER, when watching movies, if you choose to output "bitstream", which is in the "video settings" area, your AV reciever can often up-mix 2ch audio into 5.1 and 5.1 into 6.1/7.1 from many of the popular DTS/Dolby formats, provided the source material uses it. (which it almost always does nowadays).
Video games are a different beast entirely and while they command a truly prolific market share of media, their audio standards are various. you should almost always expect LPCM output from games to be safe.
If you are interested in a super high quality 2ch stereophile experience from your ps3, you might also try to set audio settings to ouput from both the HDMI and the SCART/analog output connector that came with your ps3. Connect the 2ch analog outputs to your favorite input on your favorite pre-amp or reciever using your favorite cables and go from there. Make sure you select the proper input on your reciever as the HDMI will not be used in this case. The reason for this... it is from this output that you will get the highest 2ch sample rate/size you can get from the ps3. why this is I admit I am not sure. Give it a shot.
May your angels always be stronger than your demons.