UPDATE: A pretty big correction is in order. Earlier today I was given a picture by a source who had access to a National League ballpark and who was told by a ballpark employee that the photo below was, in fact, the replay machine used at the park. I have been told by Major League Baseball sources and sources from the ballpark in question that my source and the ballpark employee was mistaken. The picture is of a video monitor used by the groundscrew, not umpires for replays.
Major League Baseball shared with me photos of the actual replay monitors used. While I was told that I am not permitted to use the photos, I can describe them: they are 19″ Panasonic flat-panel monitors. I am told that they have HD capability. This conforms with what was reported about the state of the replay equipment used by MLB in the wake of the Angel Hernandez-Adam Rosales home run incident a couple of weeks ago. While there has been some question about whether the system was uniform, I am told by MLB that it is.
There is still much to be said about the state of replay in Major League Baseball and whether the current rules and procedures in place are sufficient to get the calls right. But it is not the case, contrary to my earlier post, that any park is using non-HD monitors for replay review.