The warranty doesn't cover accidental damage. You'll have to purchase an out of warranty replacement phone, which costs $199 (US) for the iPhone 4S. However, if the screen is defective they will replace the phone for free if it is still under warranty.
Bummer. Be sure to back up your phone, import your photos to your computer and sync your contacts with iCloud (or back them up with Outlook or another service) before taking it in for replacement. (On the bright side, $199 is a lot cheaper than an off contract replacment would be.)