I can't think of anything that could have lead to a copy of my wallet.dat file getting out. The iMac that I'm using never leaves my desktop, my Wifi is secured with decent security, the wallet.dat only gets backed up to my Time Capsule.
I really appreciate the comments and suggestions by the more experienced members. I'm not going to be putting any more BTC in this wallet. I'm going to be much more security-conscious when I set up the next wallet.
One of the ways that I believe someone could have hacked it is by connecting to my wallet client via a socket. I ran a little Perl server that listens on port 8333, but nobody connected to it. Unfortunately, that was after I closed down all incoming ports on my router firewall (I had only ssh, http, and minecraft ports open, and they were not directed to my Mac), so that's not conclusive.
Random8