The two models are still viewable in the Google Cache:
silver and
blue.
I noticed that the PRS-500 spec sheet already implies USB2: "USB Port: USB 1.1 or faster port," so this may not be an upgrade on the PRS-505.
I've also been seeing references on the blogs about a faster refresh rate on the display, but I didn't see anything on the ABT spec sheets about that. Has anyone seen anything concrete on this?