Before formatting, give a try to Pocket Mechanic 1.49 (
http://www.pocketgear.com/software_detail.asp?id=12178 ) to repair the card. This tool is far inferior to
chkdsk on the desktop, but you don't have anything to lose if it meshes up the file system. You have the chance that it will work OK. (For me, it worked about 20% of cases. Again, chkdsk with a dedicated SD card reader is recommended.)
There is another "chkdsk-replacement" for PPC, Storage Tools (
http://www.softwinter.com/storagetools.html ), but I had even worse results with that. Actually, it NEVER worked right with me.