Links to Rafe's extensive Nokia 5800 XpressMusic coverage: Intro and Hardware, Multimedia, Software and Compatibility.
Here's my test clip and photos. Video is shot on the 5800 XpressMusic at 640 by 352 pixels at 30 frames per second and is of quite decent quality. This is the first time a S60 phone has offered 'wide-screen' aspect ratio video capture and Nokia have done a good job.
Cynics will point out that the picture is just a cropped 640 by 480 video (which the 5800 also offers), and that the sensor in the 5800 is smaller than that in the likes of the N95 and N82, but in good light conditions, footage is stunning when viewed on the handset, with that lovely wide 3.2" screen, and quite passable when passed on to other systems as shown below.
With widescreen video seemingly becoming 'de rigeur' on the likes of YouTube, this will be an excellent way of creating video content that looks modern and fresh.
In poor light, the size of the sensor really kicks in, with large amounts of digital noise in both videos and stills.... hey, that's physics!
Steve Litchfield, All About Symbian, 19 Dec 2008