If you have an iPhone 3G power adapter that looks like the photos at right, stop using it immediately.

That's the word from Apple Inc. (AAPL), which is warning users that in certain conditions those little metal prongs can break off, get stuck in the power outlet and give you a very bad shock.

According to a press release issued Friday:

"Apple has received reports of detached prongs involving a very small percentage of the adapters sold, but no injuries have been reported." (link)

The adapters were supplied with every iPhone 3G sold in the United States, Japan, Canada, Mexico and several Latin American countries (see list here). Anybody who bought an iPhone in one of those countries received the bad adapter.

Analysts estimate that Apple has sold more than 4 million iPhone 3Gs since it was introduced in July.

Along with its tersely worded safety warning, Apple on Friday announced the details of a power adaptor exchange program:

"There are two ways to exchange your current ultracompact Apple USB power adapter for a new, redesigned adapter.

Order a replacement adapter via the Web. These replacement adapters will ship within three weeks of your order, starting on Friday, October 10.

Exchange your adapter at an Apple Retail Store starting on October 10."

Apple will replace the old adapters with the one pictured here, identified by a small green dot. The old adapters must be turned in at the same time; iPhone owners who order their replacement via the Web are being asked to give an address so Apple can send them a mailing packet.

Product recalls are not unusual in the computer industry, although they usually involve defective batteries. In August 2006, Apple recalled 1.8 million notebook batteries manufactured by Sony (SNE) because they had a tendency to overheat and, on occasion, catch on fire. See here.