I've resisted XM Radio, as much as I enjoy it in my friends' cars, because I've been waiting for something like the Cambridge SoundWorks Playdock. The Playdock works with the excellent Delphi XM Roady Radio, essentially as a docking station that turns the Roady into a beach-ready boombox. You can use the Roady in your car, and then pop it out and use it anywhere inside the Playdock.
The XM Roady is a complete set for the car (receiver, antenna, cassette adapter) for about $120 and the PlayDock, which will be available later this summer will set you back about an additional $200. The XM Radio service is $9.99 a month.
Read the Playdock Press Release.
Read about the possibility of PDA-powered XM Radio at Engadget.