iPhone lock button broken - any apps that could help?

My sleep/lock button on top of my iPhone 4 is broken so I can no longer lock my phone without waiting for the autolock to kick in. Unfortunately my phone is out of warranty and I am not able to upgrade my phone until september which means 7 months of frustration.

Does anyone know of an app that allows you to instantly lock the screen?! I have found some that lock documents etc but not the phone itself.

thanks!