Fisheye lenses are nothing new, but this one is designed to be held in front of or attatched to a compact digital camera or mobile phone camera lens. Below is the lens sitting on top of a Canon Ixy 8 meg (!!!) compact.
It's a great little toy and it only costs 10,000 yen (50 pounds). The trends site mentions nothing about larger sizes, but my friend mentioned that it also comes in medium and large sizes, and thought that the larger size would cover and convert a 50mm lens on an SLR.
October 21st: Sadly these don't seem to be available to cover 50mm lenses. A shame. The three sizes available are still only to suit the smaller compact cameras, but I'm holding out for an adaptor.