I actually was looking very seriously into doing precisely this. The problem is that I have no way of automatic verification of Moneypak receipts -- I can automate everything else and go ahead and send somebody their bitcoins, but I can't automatically make sure the Moneypak was paid for and not stolen. Meaning the first day the service was live, somebody would get my account frozen by giving me a stolen moneypak :(If you have any ways around this... I'll really do it.