I dont see an issue with tethering your iphone to ipad. its not like its a computer and your running torrents and such. ATT has allowed apps like slingbox and have even ok'ed skype over 3g even tho skype hasnt taken advantage of it. So just dont be a bone head. Dont use it for 24 hours staight downloading big files or streaming files and youll be fine. the ipad uses pretty much the same amount of data for regular browsing as the iphone so I don't see the harm in using your phone to give data to the bigger screen.