Also
From the main PS3 desktop, go to the "Settings" menu, then down to the "Sound Settings" option and select it, then choose "Audio Output Settings".
It will then ask you to "Select the connector on the TV or AV amplifier (receiver).". At this point select HDMI, then select Automatic. It will then give you a list of the settings it has selected and ask you to press X to save them. The list should be:
Dolby Digital 5.1 Ch.
DTS 5.1 Ch.
Linear PCM 2 Ch, 44.1 kHz
Linear PCM 2 Ch, 88.2 kHz
Linear PCM 2 Ch, 176.4 kHz
Linear PCM 2 Ch, 48 kHz
Linear PCM 2 Ch, 96 kHz
Linear PCM 2 Ch, 192 kHz
Linear PCM 5.1 Ch. 44.1 kHz
Linear PCM 5.1 Ch. 88.2 kHz
Linear PCM 5.1 Ch. 176.4 kHz
Linear PCM 5.1 Ch. 48 kHz
Linear PCM 5.1 Ch. 96 kHz
Linear PCM 5.1 Ch. 192 kHz
Linear PCM 7.1 Ch. 44.1 kHz
Linear PCM 7.1 Ch. 88.2 kHz
Linear PCM 7.1 Ch. 176.4 kHz
Linear PCM 7.1 Ch. 48 kHz
Linear PCM 7.1 Ch. 96 kHz
Linear PCM 7.1 Ch. 192 kHz
Hopefully this makes a bit more sense, and fixes the problem.
Remember setting the PS3 to 5.1 PCM will mean that your receiver will show 7.1 multichannel as it is giving you the best sound possible.