I know that this iPhone will work on the 3G network but I was wondering if it would work on the 4G network in the future? I've read reports that ATT will start rolling out a 4G network sometime next year, whether or not it is experimental I'm not sure, but will this iPhone 4 be able to work on that 4G network? Or would we have to purchase an upgraded iPhone that is compatible with 4G? Thanks.