I have an '07 GT with 400 miles on it and am experiencing the same problem. I have 18" rims w/ BFG KDW's but no other mods. I also notice the shimmy intermittently, mostly from 50-70 mph. It's funny, because the wheel moves but if I let go of it, the car drives straight and smooth, only the wheel is vibrating slightly left and right. It doesn't really bother me, and I can't believe it's a tire or balance problem, because they're brand new. I didn't drive the car over 30 before I got the new rims/tires, so have nothing to compare it to. If it were a balance problem, wouldn't it happen all the time, at all speeds??? This is weird, but thankfully it doesn't bother me too much. However, I don't want to destroy $1000 worth of tires, so I am going to take it in to the tire shop and see what they tell me. The shop is first class, and very honest. Could it have anything to do with the WIDTH of the rims/tires? I've got 9" rims front 10" rear w/ 255/45's and 285/40's.