It is a car charger unit that has a line out socket on it for the use of connecting the WM Port equipped sony units to a car system via Aux line/FM transmitter/car cassette audio adapter. I got it at a local sony style store over the weekend.
SonyStyle.com | Sony | Car Connecting Cable
And their dock cradle has a line out.(I was going to get one but they were out of stock there)
SonyStyle.com | Walkman<sup>®</sup> Cradle BCR-NWU3