This is the new, redesigned Wii Zapper from Nintendo, for use with future Wii shooters.
Nintendo says the Zapper will be released later this year for $19.99 (probably £15 over here, although that's a guess) bundled with a first-party title that will demonstrate its potential in the same way Wii Play shows off the Wii Remote's talents. So expect it to be a simple package of mini games.
Nintendo also said that a number of games were being designed specifically for the Wii Zapper, including Sega's Ghost Squad, Medal of Honor: Airborne and most significantly Resident Evil: Umbrella Chronicles.
And here's a video demonstrating how the Zapper handles with the zombie blaster, as well as an image to gawp at below.