It seems that a small percentage of the tiny power adapters that shipped with iPhone 3G phones in the US, Japan, Canada, Mexico and some Latin American countries (i.e. pretty much country that uses our 2 flat prong style power adapters) may have a problem. Apple states that the prongs may come off and stay stuck in an outlet, and that this has happened with a few adapters.
They'll be providing new and improved models with better attached prongs and a green dot so you can tell you've got a revised one. You can order the replacement online or get one in an Apple Store. They'll be available Oct. 10th. Until then, Apple recommends not using the little buggers.