There are 250 Million users using facebook on mobile devices – getting facebook credits there is only a natural evolution of the product.
There is a project called “Project Spartan” which will be built on the HTML5 Platform that will enable it to run on iPhones and IPads. HTML5 is considered to be the coding language of the future, since it can be used on mobile devices as well.
This is all technical Chinese, the first mission of this vision is to buy virtual stuff on mobiles using facebook credits. Later on, I think it will also consolidate with facebook deals and places and allow buying physical goods.