lol
Data stream and voice stream on a 3G phone occupies different spectrum of the bandwidth, it is why you can receive a phone call while surfing the web
as for killing the contract, this may hurt you more if you got an early termination fee attached
as a form of protest this seems somewhat reasonable we are only talking about 1 hour here, sure some users will suffer slower then normal data performance but i would seriously question anyone who is using their iPhone as a host for mission critical data, the point here being is if at&t got a better network infrastructure, such a protest would not be a problem and should not come close to taxing their network