Well, I suspect that they also pro-rate the amount of data that they give you too - so if you really only turn it on for one day, then you only get 66MB of data to use for that day - and get charged some sort of overage if you exceed that.
I'm somewhat considering dropping the unlimited plans that my wife and I have in order to save $10/month and then just start paying when we actually need to tether. I've tethered for years, but at this point the only reason I jailbreak is to tether, and it's become more hassle than it is worth most of the time - I'm always behind a couple of versions of iOS, etc - and since I only really tether somewhat infrequently, I might just as well do the official method and not have to think about jailbreaking anymore. I think the $10/month savings would more than pay for the tethering the months we need it.
For people doing this, can you clarify how the data is pro-rated?
Also, does anyone know if tethering can be turned on immediately through the AT&T app, or is there a delay before it gets activated?
Thanks!