Bicycle Retailer is reporting that Lance Armstrong is going to enter this year's Leadville Trail 100 MTB endurance race. The article quotes Lance as saying “It’s a mountain bike race in August held in Colorado called the Leadville 100. It’s about an eight to nine hour mountain bike race, mostly on single track that very few people do and even fewer finish.”
This year's Leadville 100 has not been scheduled. Last year's was held August 12th, 2006. Here is a race report from the 2005 event.