I have unlimited data on AT&T and would like to use an iPhone to tether my iPad or laptop. Can this be done with an app? Thanks.
Only if it is jailbroken, I believe. If you are jb, look up TetherMe or MyWi. If not jailbroken, your option is to contact AT&T and add a Tethering Plan to your account, at last check I think it is $50/mo for a 5gb plan, month to month. Or change your plan into the Data Shared plan and get Tethering for free but it goes towards your monthly limit of GBs.
It's not possible on a non-jailbroken iPhone to tether with an unlimited data plan, UNLESS you managed to snag at least one of the illicit tethering apps that were sold in Apple's appstore before they were pulled. Even in that case, you can only tether a computer to your iPhone--not a tablet.
Yes, he could use MyWi but there's a huge possibility that he'd get caught by AT&T. They've been cracking down very hard lately on tethering.
I haven't used it myself but here's an option that bypasses the need for an app: http://gizmodo.com/5892137/itether-is-back-bypassing-hotspot-restrictions-and-the-app-store
Lucky you, can you tether that to an un-jailbroken iPad? I considered buying it, but I figured I already had handylight. Then got the iPad and found out I can't tether to an iPad with it unless the iPad was jailbroken.