Going to preorder the iPhone 4 on the 15th and want to sell my 3GS on craigslist asap. Can I just buy this att gophone http://www.walmart.com/catalog/product.do?product_id=12456294&findingMethod=rr& and insert the sim from my 3GS in it until I receive my iPhone 4? Just wondering if there will be any problems with using my sim, I assume it will be fine since it is an ATT phone.