Programs like Banshee and Rhythmbox will automatically recognize when a portable music player (e.g. an iPod, a cell phone that can play music, or one of the many flash-memory music players) is plugged into the computer and let you manage the music on the portable device and synchronize it with your local music library.
On Linux, all of this magic happens via HAL. Specifically, the file
/usr/share/hal/fdi/information/10freedesktop/10-usb-music-players.fdi (on Ubuntu 9.10 Karmic) contains the rules to recognize portable music players and define their capabilities. Since the Nexus One is quite recent, it's not listed in that file.