I have shot several MLB games using my 5D Mark II. It does not have the reach of my 60D, but I can shoot clean up to ISO 3200, which is helpful in night games. My Flickr page has a couple shots from an Orioles/As game.
Note that some parks will hassle you if you bring in big glass. For example, shooting the 70-200 f/2.8 II at Camden Yards was fine, but the Nationals staff hassled me a lot, despite that the lens is "permissible" in both parks.
To get good shots, show up early during batting practice.