Most times, you're only able to reformat by using a program like Action Replay Max or something of that sort. I know some other people have suggested using a copy of Viewtiful Joe(or is it Viewtiful Joe 2?) due to the card erasing properties I think it had.
This is one thing that bugged me to no end, once you formatted a memory card, you were SOL if it corrupted without using some third party program to be able to reformat.