They will probably replace it for free unless they determine it is something not covered by the warranty. Accidental damage such as caused by dropping something usually isn't covered by the standard warranty.
Make sure you back up your files. Although this has nothing to do with the hard drive you might wish to remove any sensitive files first, and have copies of all your files just toi be sure.
You posted on the Tiger system forum where nobody runs a MacBook Air. In future you should check you are posting in the correct forum because that will get you the fastest and most experienced responses for your issue. I requested your post be relocated.