What is this thing called an unconference? Carl from nGen Works sent over the following link: Understanding the Unconference. It offers a good introduction to the format. The biggest issue with unconferences is the expectations of the folks that have never been to one:

The biggest challenge for unconferences may be in setting expectations for the participants. Until you have attended one, it’s difficult to appreciate the improvised nature of the event. The lack of managed direction can be something of a shock if you’re used to attending more traditional conferences, and may be off-putting to some. And, of course, since everything is put together on the fly, there’s a strong chance that finding the right sessions will be hard. Without one theme to guide the happenings of the day, the overall feel can be chaotic.