ESPN is reportedly having discussions with both AT&T and Verizon about potentially subsidizing data plans for their mobile subscribers. That is, ESPN would pay the carrier so that any bandwidth you accrued on their sites watching videos wouldn't count towards your monthly data cap. Then, you'd get smashed in the face with more advertising, and the situation would be win/win for all parties involved.
This would be a huge step in the right direction for ESPN and the carriers, especially as unlimited data plans begin to fall by the wayside as new phones are introduced. It's also a bit of a safeguard, even with wifi hotspots popping up in plenty of public places, for those that are concerned about exceeding their data limit. Stuck as a passenger in a long car ride and you're going to miss the game? Throw on WatchESPN, and all will be good with the world, plus you're not going to have to worry about blowing past your 2/4/whatever gigs of data per month.
This could be something that other outlets (be it a league or a network) could look at as a blueprint for the future. I'm sure MLB fans would love to watch more full games using At Bat on their devices when they're outside of a wifi-rich area, but could be horrified of the sheer amount of data necessary to stream a near three hour game.