I wired up my N900 into my car's monitor a while back. TV-Out is one feature that seems to be limited to only a very small number of handsets at the moment. Nokia has been doing it with the high-end N-series handsets since the N95, and the N900 continues that, being supplied with the CA-75U cable required to transmit audio and video to an external screen. It is only composite video, so don't expect the quality to be fantastic, but it means you can wire the N900 up to a projector, a hotel TV for playing any video files stored on the unit, or in my case the screen in the dashboard of my car.
I don't have any integrated satellite navigation in my car, so having the ability to use Ovi Maps on the screen in front of me is a great help. Now if Nokia will hurry up and release their new, free, turn-by-turn navigation for the N900 it'll be a perfect solution. Unfortunately, wiring in the CA-75U cable to the back of the head unit has meant that I don't have a spare for other uses, but since they are very cheap to pick up on ebay that's not a problem, and I've ordered a couple of them - one for office use, and one for everything else.
I guess with a good enough connection, you could also use it to stream your favorite YouTube videos to show all your passengers, but of course don't distract the driver when doing that ;-)
 - For the record, I think it's great that Google and Nokia are giving navigation software away now, because tomtom has charging way too much for it's phone navigation up to now, and I hope this hits them hard. I would happily have bought TomTom mobile for previous devices, but it needed to be half the price, and without the restrictions on moving it between devices.