Thanks Bob for the in-depth review. I haven't yet sprung for the 505 yet, but that's going to come soon after hacks are working on it. My co-worker bought one and let me feel it up for a bit. My only additional thoughts are:
1. Cover design: Personally I loved the design of the 500's cover. It was a little tricky to get on/off, but I liked that because it felt solid. The new cover attachment seems pretty flimsy to me. I also like to fold my cover over and set it down tent-like on its side as I read in bed. I'm not sure if the new cover can handle that.
2. Direct Page Control: Call me old fashioned
but I prefer the 500's hack over the official functionality in the 505. The 505 overlays the whole screen showing the input numbers which is far less elegant than using the status bar. That said, I'm still very glad that Sony is noticing the features that are popularized here and incorporating them in the firmware.