According to the Kinect for Xbox 360 Facebook Fan Page Microsoft did establish a new world record for simultaneous participants in a virtual 100M Sprint.  They did this using Kinect for Xbox 360 and Kinect Sports:

A big thank you to everyone that took part in the World Record attempt at the weekend. We can now announce that Xbox LIVE members across the globe have set a new Guinness World Record for the largest simultaneous online 100m sprint! If you’re one of the 10,539 people that took part, you can call yourself a World Record holder!

I was one of those 10,539 people – were you?