The first BMWST Unrally happened in 2002 in Gunnison, Colorado. During the planning phase, a deliberate decision was made to keep it very casual, unorganized, and free of vendors and other commercial interests. This is in stark contrast to traditional motorcycle rallies like the annual MOA rally, and so the decision was made to call it an "Un" rally.
The typical Unrally format that's evolved since then involves an organized group dinner one night, an organized group breakfast one morning, and accomodations (hotel and/or campground) at one central location. Attendees receive a few souvenir items (typically a T-shirt and commemorative decal). No vendors are present. There is no beer tent. There is no live music in the evenings, unless someone happens to bring a pan flute or an accordion. There are no organized group rides. Organizers often provide suggested routes that people can ride at their own pace and on their own schedule, either as individuals or in groups of their own choosing.
I gather you missed this UnRally. If you're a glutton for distance, next year's UnRally is slated to be somewhere in the Sierra Nevada mountain range in July, with exact dates and location to be officially announced sometime in the coming months. If that's too far, well, keep an eye on 2011, when it's likely to move toward the east again.