Image by mindjuju via Flickr
Concept is a coding barcamp. People show up to help or to present a coding project, new or existing, in 30 seconds. Everyone spends a bit of time getting organized (determining where each group will reside to hack), then the hacking begins. Breakfast, Lunch and Dinner are provided as well.
There is also a Wiki page about the hackathon with more information. The hackathon is sponsored by XMission and Stackable.com. This sounds like it will be a fun day!