Maybe I'm wrong on this, but I get the impression that the device is technically called the "Pandora", not the "OpenPandora", and the only reason anyone says "OpenPandora" to refer to the device is to differentiate it from the music service.
So... the Pandora's name is a rip-off of... Pandora, not this "OpenPandora" app. It's the companies' names (OpenPandora Ltd. and OpenPandora Gmbh) that ripped off this app. But then again, I haven't been around long enough to know when and how "OpenPandora" came about (here, not in the OpenPandora app), so this is just conjecture.