This could be a great idea in the big leagues, especially during the playoffs. It would definitely come in handy for questionable home runs near the foul poles and plays where there's fan interference like the infamous Jeffrey Maier catch in the '96 ALCS.
Re: Instant replay is being planned for the Arizona Fall League
It is about time.
The admitted miscall on Arod's HR last night just adds the exclamation point. I hope that they take a pointer from the NFL, however. These should be "coach directed" challenges with a limit (3 is fine, like football).