Thanks to @theiphoneblog for tweeting about this. ATT is out to get more $$ from us iPhone users like we (or our companies) don’t pay enough all ready. Now they want more if you want to tether your phone to your laptop to use as a modem. But you know, this is nothing new really. For a few years we have heard about cell companies wanting to punish tetherers (yes, I made that word up) by charging them more for data use.
So now, it looks like this is becoming a reality. Please read: http://www.theiphoneblog.com/2009/01/05/att-tethering-plans-drop-week/