They having a network contracted for ATT users traveling through/visiting VT, but they do not have their own network in VT. So you can't get ATT service for an iPhone or any other phone if you have a VT address.
From what I've read, you are fine taking your iPhone up to VT if Boston is your regular home area and at least 50% of your usage is there (or anywhere else on ATT's network). Apparently, if the majority of your usage is on roaming networks (like the entire state of VT), they will shut down your service. I assume this would have to occur of several months in a row, but I don't really know.
For what it's worth, I'm leaving Verizon and getting an iPhone when the new version comes out, and I spend a lot of time skiing in VT in the winter...and I'm not anticipating any problems.