Sony announced that it's opening up the device's proprietary WM-PORT connector to third-party hardware developers via a "Designed for Walkman" licensing program, similar to Apple's existing "Made for Pod" program. Expect to see add-ons soon, beginning with speakers, microphones, and GM transmitters from Logitec in October.
More info (Japanese):
http://www.sony.co.jp/SonyInfo/News/Press/200809/08-118/index.html