I saw this yesterday while teaching a class with a Vintage 2009 Memory Card loaded with original 5D MK 1 images. (Those old tiny memory cards are good for something, eh?). What are you system details? I was going to file a bug on it but want to know if it cross-platform or Mac only.
I'm using an iMac 10.6.8 and bought Lightroom 4 yesterday and have checked for updates etc just in case.