one only need look to blackberry for an analog. (i know i know, mac forum, hear me out).
On Verizon w/ a blackberry, you can have both A) truly "unlimited" data while using the blackberry directly, and B) a "capped" 5 GB tethering plan when using a connected computer. they are quite capable of keeping track (and do keep track) of the 2 data streams. dunno how, but they definitely know one from the other.
The reason they have the caps is to keep excessive use off the network (or at least to have a means to punish you for excessive use), like P2P, torrents, streaming video, etc. You think the network can all of a sudden handle tethered phones running as broadband modems? No. You think they'll let you do it for free? i would sell short on ATT if they do.
Not trying to be defeatist, but would hate to see a bunch of people paying for a service they thought would be free.
my .02