+1. I think AT&T only gets their panties in a bunch when people with grandfathered unlimited plans abuse tethering. I recently took a road-trip vacation and did a bit of tethering while on the road. No more than 500 megs or so on my 2G plan and no one was the wiser.
I've heard that they sniff for user-agent strings in browser packets. I think this is only on the grandfathered unlimited users though, but to be safe, on my vacation, I used Chrome and the user-agent switcher
plugin to make chrome look like the android browser.