This excerpt taken from the SIRI 10-K filed Mar 13, 2006.
Portable Units. In 2005, we introduced the SIRIUS S50, a wearable satellite radio. The SIRIUS S50 allows users to capture and store up to fifty hours of SIRIUS music, or a mix of SIRIUS music and MP3/WMA files, enabling listening on the go. When attached to a home dock, car dock or executive docking station, the SIRIUS S50 provides easy access to live content, and captures and stores SIRIUS music.