I don't think it's your computer joining the network that's the issue; the network itself is likely tainted. By extreme example, a rogue cell tower was demonstrated last year which monitored outgoing SMS messages. Your phone would automatically join it because its signal strength was greater and it had all required information.

The issue with Defcon Wi-Fi is that you should assume all outbound traffic is captured. Are you sure your mail notifier, Dropbox client, IM client, etc, aren't sending credentials or some kind that can be (at least temporarily) exploited?