The N810 and computer part could in theory be possible (except for the full screen perhaps) by having the computer act as both a media server and media renderer under a upnp/dnla setup.

As for the car part, that would require the N810 to act as a A2DP and AVRCP unit for your phone or other bluetooth device. While there is bluemaemo that makes it appear as a HID device, i do not think there has been any work towards it acting like the receiving end of bluetooth music.