There may be something lodged under it. I'm not quite sure if the Apple store would be of help, but it would not hurt to bring it in, in any case.
A quick google search lead me to this demonstration of key removal on the white Macbook:
http://www.youtube.com/watch?v=E--mfTViQVM
http://www.youtube.com/watch?v=1QhlpFIrIw0
You may want to attempt this, at your own risk to check under your key for debris. It's a fairly simple process and will allow you to see if there's anything under your key, or if the rubber piece is placed correctly.
Good luck.