A Guinness World Records achievement for "most people running a virtual 100m dash at one time" will take place over Easter weekend, on April 23. According to Rare, a free Xbox Nations gamer pic should be available in the coming week. Downloading it will "automatically enter you into the record attempt."
Then, at the appointed time (listed after the break), all you need to do is pop in Kinect Sports and "complete a sprint mini-game while connected to Xbox Live." Make sure not to choose the entire track and field event from the menu or it won't count.
All who download the gamer pic and participate in the record attempt will be rewarded with the Guinness World Records avatar tee (pictured). Beyond that point, you can totally make up the reason you earned it: "Yeah, I totally got that for putting four pool balls in my mouth. You should see how many you can do!"