NOTE: These codes have been reported to only work on version 1.3 of Temple Run, also possibly only on jailbroken devices where you can get the keyboard to appear (allowing you to enter the cheats). There are no other cheats for Temple Run, so if you cannot get the keyboard to appear on your version, there is no way to get these cheats to work for you.
The original cheat was to unlock invincibility and additional characters, using the codes below while on the Title Screen:
Unlimited Invincibility: samhines86
Unlockable Characters: rxh7nigh
Edited: Jan 9 2012
Added by: MongooseGeneral Nov 1st 2011, ID#376