actually, the stones are doing the opening night, not the superbowl... and i believe that u2 did the superbowl free of charge in 2002. at least thats what i have read in either rolling stone or newsweek.
U2 gave the money to charity. I would be surprised if the stones didn't do it, it normally seems to be the big touring act of the year and U2 have done it recently (in superbowl terms)