No solution.
Apparently, AT&T decided that if they now offer "unlimited" data plans (which are not really unlimited), they no longer need to share usage accounting with us. Which doesn't make any sense. I do want to know if I have slow Internet because throttling came into effect or just because I am in a place with bad connection, and I want to plan in advance and possibly purchase additional data pack if I am close to hitting the plan allowance.
Does anyone know if they at least send a message when the monthly limit is reached and throttling begins, or they just start to throttle silently ?