Kevin C. Tofel at jkOnTheRun reviews the Nexus One dock on video:
It’s just not seamless, yet.
A quick summary: The Nexus One can easily slide out of the dock because there isn’t a USB connection on the dock: the connection is via Bluetooth. The clock app runs when docked, most of the time. Audio plays via Bluetooth, some times. The dock is $45 and you get an additional phone charger and a 3.5mm-to-RCA cable.
Android Mom also has a video showing how you can stream music via Bluetooth from the Nexus One to the dock, which is connected to her speakers.