This area is beat to death.
There is no clear answer because as ATT and others change the rules then any answer given today may be wrong tomorrow. For example, some found they could pop the ATT sim into said units and later get a nasty automatic upgrade to the plan.
http://www.blackberryforums.com/general-blackberry-discussion/190664-blackberry-bold-t-without-data-plan.html covers other angles.
The carriers do not want smart phones with no plans and they are working hard to discover anyone that has such and IN THEIR AGREEMENTS is some clause about this.
I hope this helps you understand why it's a minefield area. The moment someone finds a workaround, the carriers seem to know about it.