Apple wants you to give back your Ultracompact USB Power Adapter you got with the iPhone 3G to get a new one, the old one might hurt you. Apple announced a Apple Ultracompact USB Power Adapter Exchange program.
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. The ultracompact USB power adapters were supplied with every iPhone 3G sold in US, Japan, Canada, Mexico and Several Latin American countries.
Apple advises that users with ultracompact power adapters should immediately stop using them until they exchange them for a new, redesigned ultracompact adapter.
Replacement adapters will be available starting Friday, October 10, 2008.
More details in this Apple support page.