One of the new cool features of the Cisco 504G is support for wideband (hd) g722 codec. Note that g722 is only available with the 500 series spa phones. You spa 900 series owners are out of luck. Asterisk 1.4.x does not support g722 without patching, I've used the following patch and instructions with Asterisk 184.108.40.206 and it worked.
The one thing I noticed with the g722 Asterisk sound files is that the volume is much louder - I had to take down the volume half way on the spa504g. Everything sounded clearer but none of my gateways support g722. I know technically pots line would not provide hd quality audio but if the gateway supports g722 my Asterisk box would not need to transcode from 711 to g722. Everytime there is transcoding you lose quality, create latency, and slow down your system. These are all bad things and should be avoided.
Now I just need to configure another phone with g722 and do some test. Perhaps play some music and see if I can hear the entire range of sounds.