Physically, the only difference is the HP branding on the so-called hPod's backside. There are three more main differences:

Windows-focused help
The contents inside the HP-branded box include Windows-centric user guides, as well as a quick-setup poster.

Pretty tattoos
HP is selling printable tattoos that wrap around an iPod to give it a personal touch. Of course, these tattoos can be used on any iPod regardless of brand.

Extra support
While both Apple and HP offer a one-year full warranty, HP extends Apple's 90 days of support to a full year.

The only advantages of the iPod from HP are its friendliness to Windows and its extended support.