We can't cut our hair for a year (as of May 26, 2010). The consequence for violating the bet is whoever cuts their hair has to shave off their eyebrows. And they will be shamed!
I've made these kind of bets before, but have never followed through with them. The problem has always been that it's a bet only against myself. Now it's between two people, and Mark is serious. Hopefully this one works out for the best!