Without an understanding of how ATT is tracking....you cant hide the activity.
ATT does have tethering plans.
My concern with switching (from unlimited), is that I just dont have a clear understanding of how the 4S with its higher data speeds, siri...etc will affect data usage.
Also, tethering means I want to connect other devices and they will consume data.
Hard to know how far the 4GB will go.
ATT and all the other carriers are stingy with data and overcharge in a big way.