I know when you do it online, you're guaranteed that amount for thirty days. However, I need my 3GS up until I get my 4; but I'd rather not charge anything to my card. So, I'm planning on trading it in, in the store, on launch day and then walking over to the Mobile section and buying the 4. But I'm afraid their $269 offer won't last, and will drop significantly in the coming days. Any suggestions?