There's only 8 teams in it right now, but there should be more!
the official description:
Join the friendly competition - workplace against workplace - to see who can bike to work more during the month of September. The workplaces with the highest percentage of commutes by bicycle during the month win!
Whether you're a rider or an employer, whether you're thinking about trying bike commuting for the first time, or you're an experienced rider, the Challenge is for you.