Agree with anglefire, formatting a card in a computer give you two choices either a quick format, which will only delete the Fat file, so images can be recovered by software, or a low level format, which takes a lot longer but deletes the images as well. Either method will not cause problems to the card and I've used both on occasions without problems. Formatting a card in the camera only deletes the Fat file and not the images themselves, they just get over-written as you take new images.
