I have never never reformated a card after the first time and have never had a problem. But do make sure that you turn the camera off before removing the card also remove it from the computer in safe mode.Do you always reformat your card after you download your images to your computer? If you don't do in-camera formating after each download some very strange things can happen.
+1 to Mojo's and Phil's comments. Always format in camera. If you format two times in a row on Nikon's, it does a low-level format. Which body are you using?
Ed, JOOC, do you have a reference for this? It's not mentioned on the Nikon website or the D90 manual.If you format two times in a row on Nikon's, it does a low-level format.