5.1 compressed PCM is not really an accurate name - PCM is inherently uncompressed, and if it's compressed it's no longer PCM.

I haven't heard the new formats (DD Plus, TrueHD, and DTS HD MA) yet to know how they compare to DD and DTS, but there's certainly potential there and reports have been good. My HDMI FAQ goes into some detail on these formats. Keep in mind that this HDMI audio (multichannel PCM or bitstreams of the new formats) is only different from the optical connection for Blu-ray discs. CD's, DVD's, and games will sound exactly the same over HDMI or optical.