That's something that I've struggled with as well. I don't think it's a cell phone or a PDA, but it definitely would be considered a mobile device.
For now, if you are considering adding one to the site, don't worry about getting it classified just right. We're trying to adopt a new "bottom-up" paradigm in which we add products first and classify later.
Also, there will be some interesting updates to the site coming soon dealing with these issues.