Yep, I'm in central Kentucky and was having this exact same problem. When I brought up the Maps app it would show my location as being about 85 miles west of where I really am. I tried the turning off 3G trick and it worked...but that's silly. It started about the same time as everyone else here has described...about 2 to 3 weeks ago.
Now, though, it seems to have fixed itself. Hopefully AT&T has got this sorted out as I use my GPS at work nearly everyday, and I didn't want to have to go back to using my Garmin when using my 3GS is so much easier.