I don't understand why so many people are bent out of shape about this. If the new plans aren't for you then stay with what you have. If you're a light user, switch. My wife uses 350mb a month, I use about 1.5gb. I'm not switching but I might switch her to 2gb.
At&t has been promising that "tethering is coming." Now that tethering has been announced, it is accompanied by a reduction in allowable usage that will seriously undermine the utility of the service.
The comment that 65% of customers will benefit from this is illusory because 100% of customers will have to pay more if they want to tether, and 100% of those customers that are paying more are now encumbered with a new limitation on their service.
It does not appear to me from the OP that there is an option to stay with the current $30/month plan and add tethering. If I am reading that wrong then I guess it could be worse, but it is still bullshit, because if at&t is distinguishing their price plans based on bandwidth usage, then device selection should be irrelevant.
How hard would it be to simply have a per megabyte or per gigabyte option? That would completely obviate the need to distinguish between direct usage and usage by tethered devices.
Paying more to get less = customers getting "bent out of shape."