During the recent London riots, a lot of cellphones were stolen from various stores, and many of them are now popping up on the internet for sale. Well, you probably don't want one of them, as none of them are going to work.
U.K. carriers are examining their stock records and gathering up the hardwired IMEI numbers for every stolen handset. They're then making each of them permanently inoperable on any network. That should leave thieves with a lot of very shiny paperweights. So if you're in the market for a new phone, especially in England, it's probably wise to go to the store rather than try your luck on the internet.