It is now all but inevitable that baseball will begin to use instant replay during the 2008 season.
While it sounds like replay will only be used to determine home run calls at first, it is not difficult to imagine that a successful “test” through the end of this season could lead to more reviewable plays by the time teams take the field for the 2009 season.
It remains to be seen how replay might impact baseball, though smart money is on making games longer, though I don’t think the potential breaks for an occasional home run call will be the end of the world (though it will probably lengthen games enough to undo all of the gains made by the new directive to enforce the 12-second pitch rule).
The main argument of those who are in favor of replay is that umpires should use all of the tools at their disposal to get the calls right.
The NFL has had some form of instant replay since 1986, with the current coaches’ challenge system taking effect in 1999. A few observations from the football games that I have watched over the years:
1. There is no television moment less compelling than a referee sticking his head under a video replay hood, and
2. Somehow, even with the instant replay system in place, calls are missed.
Someone who likes football better than I should do a study on the subject of instant replay and correct calls. I don’t know what the numbers would look like, but it doesn’t seem particularly rare, based on my observations, for a call to go to replay only to still be missed (or at the very least bad calls are upheld due to a lack of “indisputable video evidence”).
I am willing to be proven wrong, but it all just doesn’t seem worth it at this point. For a handful of correct home run calls a season, baseball fans and players will be subject to delays, technology problems, and arguments over replay usage.
Coffeyville Whirlwind 
I imagine if the only disputed calls are home run calls, and two cameras were put on the bottom of the poles shooting straight up, I’m not sure how a call could ever be missed. In football, there are an infinite number of angles one could look at a play, and the football can always be hidden from view by bodies. There should be nothing to hide the view of the baseball from a well placed camera.
And how many games do we actually see a disputed home run call? One in every twenty? I’m not sure this would have a great effect on time.
I don’t think there would be too many disputed plays, though I do think that, with replay as an option, you’ll the incidence of “disputed” home runs go up. Still, it wouldn’t be a huge number.
My point is that I think that the powers that be are going to a lot of trouble to institute a replay system, that, in its present form, won’t change more than a handful of calls a year. To me, any uptick in game length is too much for that outcome, even if you ignore the fact that it is likely that the scope of instant replay will broaden significantly to include more than home run calls in the near future.