Apple has issued a recall for the "ultracompact" USB Power Adapter that ships with the iPhone 3G. A support page states:
"Apple has determined that under certain conditions the new ultracompact Apple USB power adapter's metal prongs can break off and remain in a power outlet, creating a risk of electric shock. We have received reports of detached blades involving a very small percentage of the adapters sold, but no injuries have been reported."
A warning reads:
"Users with ultracompact power adapters should immediately stop using them until they exchange them for a new, redesigned ultracompact adapter."
Apple also says that users should charge their iPhone 3G by connecting it to their computer with the USB cable that came with their iPhone or by using a standard-sized Apple USB Power Adapter (with fold up prongs) or with a third party adapter until they can receive a replacement.
Replacement (safe) adapters have a single green dot on the bottom, as pictured in the upper-left and in this document