Think of it this way:
Would AT&T charge a tethering fee to put your Atrix in the multimedia dock? All that is, is a display output and USB port, which you can use to plug in a keyboard and mouse, etc. The answer is no, AT&T would not charge a tethering fee for that.
Well, the laptop dock is just a display and a keyboard/mouse. When you use it, your phone is not providing data for any other device. That is not tethering and AT&T would not charge a tethering fee for that.
Honestly I'm surprised people are even asking this question.