Apple announced last week that it will be recalling the ultra-compact power adapter that is provided with the iPhone. It turns out that with the current design, the metal prongs can break off and remain in the power outlet, turning into an electric shock hazard. (These adapters were sold in United States, Japan, Canada, Mexico and certain Latin American countries).
The recall begins on October 10th in the U.S., at which time you will either be able to go to the Apple store or order a replacement via the web.
Here is the official press release from Apple.
This is how the replacement adapter looks. If you have one that already looks like this - you don't need to participate in the recall program. (Notice the green dot. It represents the redesigned adapters.)