Thanks to everyone for their replies.
I called ATT relating to be able to switch tethering ON and OFF. The rep mentioned that you can do it either online or by calling in. One thing that I found interesting is, say you want to go on a 2 week trip. You can switch the tethering ON then turn it back OFF when the trip is completed. The charge for tethering would be prorated so you would only get charged approximately $10 (for the two week period).
You do need to be careful if you're doing this though--they may prorate the data as well as the actual charge.
Case in point--I opted for their outrageous international roaming setup last year for a one week trip to Ireland and set it up online. Nothing in my notifications showed the prorating. Watched my data usage like a hawk, and did NOT exceed the 20 MB plan limit that I'd requested. Just one small problem...my billing period fell right in the middle of that trip, so they'd prorated that 20 MB for the 4 remaining days of the period. It came out to some ridiculously low number for those days, which naturally I'd exceeded, even though I'd stayed under their overall limit. It took a rather lengthy and heated conversation with them to get the charges bumped back down.
I'd never simply trust AT&T on something like that type of plan switching again. I've had great service with them over the last eight years, but that incident made me a lot more wary. Especially for those of us who would have to give up unlimited data to just to tether officially.
(And yes, the apps that allow tethering unofficially all require a jailbreak, unless you already have HandyLight, which snuck briefly into the App Store. It's long gone, obviously.)