Well it has been quite long when apple discovered weak signal strength in AT&T iphones but up till now it has not been able to find out the exact reasons for the weak signal strength.
Not only AT&T phones are facing this problem but also by other networks/countries.
Here are some solutions which can eventually boost your iphone signal strength :
Reset Iphone : Press Home + Sleep button simuntaneously.
Reset Network Settings : Settings -> General -> Reset -> Reset network settings
Reseat Sim card
Replace Sim card
Give your iphone a full charge.
Third party Signal booster.
Attach tape to your sim card (As per some users)
Hopefully Apple with come with a firmware soon.