FWIW, I've seen the same thing with a number of files that worked fine in the past but recently (post LR2?) started showing this same kind of corruption. I havn't tried to narrow it down, but I do know that some files that now appear to be corrupt were the same files I made prints from back before Lightroom was available.
So I spent a little time tonight and finally narrowed it down. It appears that the only corrupt files I have are from the secondary backup location I use when importing photos into Lightroom. One bit of weirdness in my setup: my backup location is a network drive, mounted through Windows file sharing, not a physical hard drive connected to my laptop. Maybe a bit got corrupted on the way over the network? Seems fishy, but as far as I can tell, only my backups have been affected. I discovered this because I was importing everything into a new Lightroom library on a new laptop and didn't have easy access to my master catalog, so I just sucked in the backups from the network drive.
I can confirm that my local copy and the copy on my backup server have the same contents (at least according to an md5 hash), so it looks like something just went wrong when Lightroom was making the backup. Or my file server is totally screwed and randomly corrupting files.
Either option is no fun.