I have two phones on an ATT Family Plan, a Motorola V3xx and an LG Shine. Each of them has a $20/month subscription for "Media Max 200 Bundle", which consists of unlimited Internet Media Net service plus 200 text messages per month. Both phones are tethered to their respective PC's. One was used as its PC's only Internet access for several months until recently. The other is used as a periodic backup to a sometimes unreliable ISP connection, and for access when traveling.
The maximum usage of either of these hasn't exceeded 2 gigs in a month, so I can't know whether ATT would catch them at 5 gigs, but they surely haven't caught either one in over a year of moderate usage. It's fairly clear that the Media Net service is only intended for use with the phone's internal browser, but ATT's web site still carries an old Cingular instruction sheet for setting up the tether connection. Where 3G service is available, the connection appears to operate at 3G speed.