I have a similar problem.
I have a PS3 Slim and a Denon AVR-X500. When I play games like Tekken 6 (which has Dolby Digital audio), the receiver displays Multi-Channel Stereo, instead of Dolby Digital.
Sa Sound Settings ng PS3, naka-set siya to HDMI Automatic so it should be able to decode the signal properly.
Is there a way to get the PS3 to bitstream audio?