I might have a clue to add...
Sorry I didn't see this thread earlier, but better late than never.
I had a similar, but less severe, experience at the rally. My wife has a very specific way of organizing her credit cards and driver's license in her zip-up-wallet, which she keeps in her purse.
During the rally she opened her wallet and was convinced someone had gone through it. Every card and her driver's license was misplaced and in the wrong pocket, though none were actually gone.
My first thought was, "Shit, someone trying to get enough information to steal her identity, but they didn't take anything to keep us from immediately canceling/closing accounts."
I started watching all her/our accounts very closely after that, but nothing has happened...yet. Curiously, she tried to use one of the cards in question at Fed Ex last week and it wouldn't approve. I immediately called Visa but there were no unauthorized charges. They couldn't explain why it hadn't been authorized (it was still "pending" in their records, but she just used another card to pay for it -- the charge was like $6.81). I think one reason we haven't had a problem is that I'm the primary name on pretty much everything, so even if this person has her information they will ultimately be asked for my information if they attempt to move funds or open accounts. At least, that's what I try to tell myself...
Here's perhaps the most interesting point -- she noticed the unauthorized "rummaging" after our night at the Petersen museum. She left her bag unattended while going to the buffet tables, and there were periods where neither she, I, nor anyone else was at our table because everyone was doing their own thing (just like at most of your tables, I'm sure).
My only guess is one of the service staff at the dinner -- but again, that's just a guess based on when she noticed that someone had gone through her bag and when she'd left it unattended.
Not sure if this helps. I'd almost fogotten about it until I saw Chris' email. Sorry to hear about it, and now I'm nervous all over again...