Find My Phone feature that all iPhones have installed?
With it you can track the location of your iPhone and even lock it down. I believe it is also possible to remotely wipe the iPhone.
my iphone 3 was just stolen and i have no lock on it. i already reported it stolen but my concern is that my email will be visable to the theif. I changed my aol email password from my home computer is that enough to lock out access to my stolen iphone email account?