Peter Gammons was on his weekly guestspot to Mike and Mike in the Morning a few days ago, and was asked about the chances for instant replay in the MLB next year. He stated that it IS likely that MLB would probably give it a try in the Arizona Fall League next year.
He also said that he only sees them using it for homeruns ONLY!!!
I think this is the correct move. Homeruns are hard to see around the foul poles and when it hits just over the wall with fans being in reach and cloggin up a clear line of vision.
I would hope that they would NEVER use it for issues of balls and strikes, or baserunners being tagged out. MLB is concerned about the length of time for a game, and I agree with them.