_MOD Edit- data used be data one could use it however one pleases, and then iphone came along and ATT asked them to selectively allow tethering on some data plans. so that they could charge more for tethering.
i think the back end of it is implemented using, APN types. Vaguely remember able to tether by setting APN type as "Default, tether". doesnt work anymore, atleast in mycase.
the APNs on my phone are locked, even if i make new APNs they keep switching back to the default ones.
since my phone is rooted there should be someway to delete network specified APNs, and use custom APNs
my guess is this should serve fine and we could just use the default android tethering on rooted phones