You can set a passcode and after 10 failed attempts it wipes the phone. I don't think theres an app that will ever do that because I don't think the SDK allows apps to touch any data other other then the stuff it creates.
My iPhone was stolen, i used gmail as the email program. bought new phone today and tried the following with success. i sent myself a couple emails and updated them on my new phone, i then went to gmail on my pc and and selected them to be archived. when i pushed for new emails on iPhone the emails were gone. i selected all emails ever sent to me through gmail and archived them, when i pushed for emails on my new iPhone the 16 emails i got today were gone. i have to assume that all the emails on my stolen iPhone were also removed.
In response to mugdiller. I think that if u Activated a new phone. The old one stops working thereforee no Internet to update or make changes to the emails on your phone. All the emails would stay on the phone. Until that person tries to sync onto a computer and needs to wipe it clean.
thanks bwise05. i should have done it before att deactivated sim card. local att store mislead me saying once they 'burn' sim card phone is useless and no info obtainable. i changed every password i have and don't think they will be able to do much with the emails.