I think keys should be removed from the inventory screen all together. They just get in the way. You pick up a key and you keep it forever (or as long and the game lets you). And like diabolus said, have a screen that lists which ones you've found and what they unlock (if known).
Deus Ex used this feature well. There was one item called a Nano-Key (I think) which you programmed with key shapes that you found. The Nano-Key remembered which lock required which key. It just made things easier.
