Just want to get on the radio? If you're at a security conference, and you've got a ham radio license (of any class), grab your radio and set it to 146.580 simplex (no offset). Say hello!
There's a lot of ham radio operators in the security community. Standardizing on one frequency lets us find each other easily. This frequency has been in use at DEF CON since at least DC16, and probably much earlier (and at other conferences as well).
A backchannel to the conference; a good way to find friends (old and new) that doesn't rely on sketchy WiFi.
Try again later! Sometimes people turn their radios off when they're in talks, but they usually will check in again later; you might just listen for other hams by keeping your radio on.
No; in fact, there are more hams now than ever before, with over 700,000 licensees in the U.S. alone in 2011. In the security community, people use their ham radio licenses all the time, whether to read RFID at more than 400 yards, run WiFi at 15,000 times the "normal" legal limit, or just mess about with radio waves.
Then you should get one! For an all-around great radio for an insane price, the Baofeng UV-5R is a pretty great deal (often less than $30 on Amazon, with Prime). If you want to use it throughout the con, an acoustic tube headset (a la the TLAs) is useful.
Then you should get one! We're giving exams at BSides Las Vegas 2015, but you can find exams almost anywhere in the country, at least once a month; check out the exam page for more information on exams generally, and how to find one in your area.