Already on me.com you can go to account settings and find the new Find My iPhone feature. I just tried sending a message to my phone "Please call for reward for return". It appeared within 5 seconds! I am not sure if this works using Wifi (Internet) or some backend message over the carrier (perhaps specific text message) - but it works.
Theres also an option to remotely wipe your iphone if you really do give up on finding it.
Update: To allow full mapping location the mobile me setting is on the iPhone under Mail/Contact Settings. Slider there to turn on find me service.