It's a Tethering Plan...and yes when tethering is made available by AT&T there will be additional charges associated with it.
(From AT&Ts site forums):
If your wanting to share/tether data from your phone you can use the DataPro Personal with Tethering plan $45 month that includes 2GB data month (additional data is $10/GB)
You can get a dedicated USB card and get the DataConnect $60 plan that includes 5GB data month (additional data is $0.05/MB)
From your Wireless Service Agreement - Data Service Terms and Conditionshttp://www.wireless.att.com/cell-phone-service/legal/plan-terms.jsp
Furthermore, plans (unless specifically designated for tethering usage) cannot be used for any applications that tether the device (through use of, including without limitation, connection kits, other phone/smartphone to computer accessories, BLUETOOTH® or any other wireless technology) to Personal Computers (including without limitation, laptops), or other equipment for any purpose.
I cannot confirm/deny any of this information, because AT&T's website is down...