Just a clarification... I talked to a friend who also enjoys tethering and wireless hotspot. But his Android is from Sprint, and Sprint disabled the built-in feature. He had to root his phone and install a tethering app. T-Mobile could have chosen to make us jump through the same hoops, but didn't.
No idea what the combined AT&T&T's policy will be -- but unless they do deep-packet sniffing, there's likely no way for them to tell an active Android data user from a tethered data user. (Though I wouldn't put it past them -- I've heard stories of AT&T going back months to sock a customer for tethering usage back in the dumbphone days.)