I want to say that I remember reading about Skype (or another VoIP provider) getting a phone that could log in to hotspots, so you may want to keep searching.

Otherwise you need a pretty fast PDA for Skype. My Axim X51v works pretty well, but Skype really slows it down. Other issues related to your questions: Axims don't do USB. If you want a headset you can get a regular one that plugs into the headphone jack (with a cheap adapter), or you can get a bluetooth one.

The Axim will take your SD card, but they don't have much built-in memory, so you wouldn't be able to backup very many pictures. So far, I've found no good way of sending/receiving files from the Axim over WiFi (to back up your pictures to a third location), but there's probably a way.

Bottom line, though, I think buying a PDA just for Skype seems a little excessive.