OK, so this is almost certainly a scam, but perhaps it's a scam that'll pay off. I'll certainly post and let you know.
At FreeMobile4U you can sign up to a list for the grand sum of £20, which, when you reach the top of it, will earn you a free gift - in my case a 40gig Ipod.
I'm very unusually gambling that £20 that this is a working scam... if you gamble with me, quote my ref number 4140. :) This info is being sold for lots of cash at Ebay - don't on any account pay for it. ;)