Also to test the answer it was corrupt before it went into Lightroom. I always copy the raw .cr2 file to my laptop and then to a external drive from the laptop directory, not from the compact flash. I looked at a .cr2 file with Photoshop and it was fine. Then I imported that same file with all of the other .cr2 files in the directory into Lightroom 4 and it was corrupt along with many others.
Perhaps you could upload this test photo to a Dropbox account and post the link here, so others could try to replicate this (since it has happened to you on multiple computers). Also, post a screen shot showing what the corruption looks like.