If you own the latest hardware release of a iPad, iPhone, or iTouch you can sign up for Apples mobile me service for free.
You can log in to mobile me from any internet connection and send a message to your iPad, iPhone, or iTouch.
If you left your hardware somewhere this is a great way to either find it or have someone contact you.
It will play a sound for 2 minutes and post the message on the screen even if the hardware is locked.
I just set up both my iPad and iPhone 3GS on this service.
MobileMe Find My iPhone Web Page:
Find My iPhone Screen Shot: