I used to have one of these. It was functionally identical to a regular iPod. IIRC they used the HP branding to be sold in retail stores where Apple had no presence at the time (Best Buy, etc).
Radio Shack used to be a big distributor of these. I never quite understood the idea behind them - I guess Jobs had a soft spot for HP since that was one of his first employers.
My first iPod was a 20 GB 4G iPod+HP. Loved it, but badly mistreated it and it died a very premature death. The only differences were the branding on the back plate, and the same in the legal info.
I'm curious. I have one (20gb HP iPod) that's not working and will be looking for repair services online, especially to replace the HD and bring it back to life. I don't need all the colored bells & whistles to enjoy my music from it. I miss its simplicity, to be honest.