Usually with disks, the content of the file remains but the entry in the index is removed. Programs exist that can scour the surface and look for missing fragments. I don't know if that applies to a memory card though but you can always try. Obviously it is vitally important that you haven't copied any new images to the card because that will overwrite what was there before.
I don't know which programs to use but perhaps http://users.erols.com/gmgarner/forensics/ [Broken] would work.
This product worked for me in recovering deleted data from a camera flash disk, but I don't remember if the disk had been formatted or the data just deleted. http://www.softwareshelf.com/files/products.asp?ID=11 [Broken]