One difference that you may find important--I was at the Genius Bar at my local Apple Store with a small issue with my own iPod, and the guy behind me also had an iPod to be looked at. The Genius told the guy, who had an HP model, that he couldn't make the repair/replacement that was necessary, that it needed to go to HP. He did find the 800 number for the guy, and sealed the case of his iPod where it had started to open, but apparently could do no more.
I was unaware of the HP model up until that moment, and I am not sure if what I observed would be true in any Apple Store, but I just thought I'd share my experience.
HTH,
AgFrost