There was nothing in my contract that said anything about a 5GB limit.
This is one of those things that's preached as gospel on the internets, but has no evidence to back it up.
I've had over 5GB repeatedly, and never heard one thing about it from AT&T.
I did get one of the letters suggesting that I tun WiFi on on my phone (it already is, there aren't that many WiFi spots here, and my 3G service is usually faster anyway).
There is not a 5GB cap on Unlimited data
Unless your contract is different than mine, and I don't know why it would be, it is clearly in the contract. Go to
www.attwireless.com and log in, click on "My Profile" then on "User Information" and there's a downloadable PDF file that is your contract. Once open, run a search for tethering or some of the stuff I quote below.
It's not a cap, but I read my contract and it said clear as day that AT&T reserved the right to add extra fees if I go over 5 GB of data. Here's the direct quote:
"If you are on a data plan that does not include a monthly megabyte allowance and additional data usage rates, the parties agree that AT&T has the right to impose additional charges if you use more than 5 GB in a month. Prior to the imposition of any additional charges, AT&T shall provide you with notice and you shall have the right to terminate your service."
It's near the bottom of page 11 on my contract. Now I didn't read all 20 pages of my contract, so I suppose this is in context of something else, but I don't think it is. This is the second time I've looked it up. It's not going to keep me from using MyWi because it's an awesome app and I have yet to go over 5 GB, but it is, in fact, in the contract.
*EDIT #2: I think this insert in the contract is simply to give them a course of action if it comes down to it. Before I installed and started MyWi I did ample research and visited many forums asking questions. Many of the posters on the forums said they used it extensively and routinely got in the 5-10 GB range, and they never heard from AT&T.