I have a buddy with 2 SANDisk cards, a 256 CF and a 128 SD. Hasn't had any problems with them up to this point (about 6 months), but just this last week he was trying to move some large files on to both cards and neither of them would allow him to write to them. He then tried smaller files just to see if it was the file size and still no luck. Finally, he tried to format them and he couldn't even do that. It seems the current content is locked on to the cards forever.
Last time I spoke with him he still wasn't able to resolve this issue.