I just got this from the Montana Trail Vehicle Riders Association. Every year they put on a huge ATV Jamboree called the State Ride and from what I've heard they've traditionally been a really great time for the whole family. I've never personally attended one, I generally ride my ATV in order to get away from large crowds not to be a part of one, but if large gatherings are your thing you may want to check it out.
At any rate the MTVRA does a lot to support the "war effort" when it comes to keeping the trails open, so anyone that rides should show them at least a little support as well as the FFOR and our own Treasure State ATV Association, another is the Blue Ribbon Coalition which works on the national level. The more turnout that events like this draw, the better case these groups can make when attempting to demonstrate the popularity and significance of motorized recreation.
The State Ride is usually held in a different location every year and even though I haven't done any serious riding in the Pipestone area, I have poked around up there a little and I can tell you for certain that the scenery is second to none and there's something up there for everyone from the rankest beginner to the most seasoned thrill seeker.
Anyway, just thought I'd pass the message along in case anyone is interested. Click on the pictures for a larger view, and if you have any questions contact the MTVRA.