Since the beginning of DefCon, there has been a significant concern about people being able to make counterfeit badges. Let's face it, this is a pretty tech savvy crowd, and you don't want something that somebody could easily duplicate.
The badges have gotten more elaborate over the years. Here is this years:
The badges have a scrolling LED display with two conductive buttons (like an iPod). The user can change the text displayed, as well as the scroll rate. It has a freescale microcontroller and a bdm port that can be used to reprogram the device (assuming you have a BDM cable).
In the spirit of open source and the ability to hack the device, the firmware source code and gerber files for the PCB are on the DefCon CD (it can be compiled with the freely available CodeWarrior toolchain). It also has in the PCB layout unpopulated ports for accelerometers and a ZigBee 802.15 wireless interface. Very cool.