Part extension cable, part adaptor; this offering from Apple will allow you to use your existing 30-pin cable or device with their new line of Lightning-equipped ports. This cable will probably also come in handy with bulky cases or accessories that may not allow the form factor of the basic adaptor to fit snugly into the port. Details from the Apple website are below.
Use this intelligent adapter to connect your 30-pin accessories to devices featuring the Lightning connector.
This adapter lets you connect devices with a Lightning connector to many of your 30-pin accessories.* Video and iPod Out not supported.