Unfortunately, you cannot have true virtual currency. The current developer contract prevents In-App purchases from being used for any stored value or intermediate currencies. It would seem inevitable though that Apple will eventually open the doors to allow this. Some developers have worked around this by offering separate versions of their apps that include a certain number of "points", which essentially amount to virtual currency. Also, Ngmoco's Eliminate is using In-App purchase in a way that equates to pay-to-play using a virtual currency.