The best way I've seen this implemented is as a pre-determined setting before a game is accepted. For example when you put out a seek, you might specify time, color, rating, and number of take backs allotted to each player. If you press the take back button, it automatically takes your last move back (even if your opponent just played in which case two half moves are taken back).
But then in blitz someone could use two takebacks in a row to seriously alter the position and screw with your head if you only have 5 seconds left. If it gives you your time back too, then there's some abuse issues there as well... especially on the turn-based side.
But problems like this aside, it would make this site too casual in my opinion. Even sites that cater more toward kids/beginners like Yahoo! don't allow take backs. I just don't see them implementing it here.
Dear sir/Executives of Chess.com / players /
At times a good player looses his game by a move by simple overlooking or in a time preasure. why not our chess.com introduce takeback option with the permission of opponent ?
K.Thirukkalathy