Apple has an exchange program for the new ultracompact power adapter that Apple has been shpping with the iPhone 3G and as a separate accessory.
Some of the adapters Apple shipped may be a fire hazard.
Apple will replace the adapter free of charge. You can go to a nearby Apple Retail Store, or order one via the web.
If your adapter has a little green dot sticker, it is one of the new ones, and does not pose a threat.