I have received several of these letters in the past. The e-mail idea is new though! The letters which I have received have claimed to be from Government officials in Nigeria wishing to transfer excess profits from government contracts to the UK. The letters have requested me to send two sheets of my company's headed paper left blank other than a signature at the bottom !! Presumably, the blank headed paper becomes an instruction to the bank to transfer funds out rather than in.
I did hear from my bank that there are instances where people have responded and had their bank account cleared out.