How serious is the 5D Mk III "corrupt images" issue?

I've seen a few posts on this forum and others about some corrupted RAW images probs (happened to my Nikon D40x and Nikon said it needed a new sensor....).

Is this a known issue, a firmware thing, a software thing, an isolated prob here and there? Do we know? Should it play any part in my current Nikon vs Canon D800/5D decision-making process? Put it this way - the Nikon problems have certainly had an impact on me as I have experienced some of them and was hoping the grass was greener (rarely is!).