Hi there, and welcome to the Q&A Forum!
Sorry to hear about the problems. I think the only way to find out if it's the card or camera is to try a different card in the camera.
How does the problematic card behave when you stick it into a card reader? If acts normally in a reader, I'd suspect the the camera but would call Nikon tech support to make sure.
I don't know that there are any signs of an aging memory card, except ones like you mention. Then, unfortunately, it can be too late.
SanDisk has a very good photo recovery program that works with Macs and PCs. It comes free when you buy their Extreme III memory cards. Funny thing though, it comes on a mini-cd and doesn't fit in Mac CD drives that don't have a tray. My guess is that if a Mac user contact Sandisk, they'd tell them how to get a copy that works. I've inquired about this with SanDisk and will post my findings when I hear from them.
http://www.sandisk.com/products/catalog(1186)-rescuepro.aspxIf you do a search, you may be able to find additional recovery programs for a Mac.
Good luck, and let us know how it goes.