Though a vapor barrier & an air movement barrier are somewhat related, they are 2 seperate issues, both of which need to be addressed. People get very one dimensional in thinking that the vapor barrier IS the air barrier. Though it can be (as is the case of poly), it doesn't need to be (As in the case with Kraft & finished drywall, the kraft is the vapor barrier & the drywall the air barrier).
To put it simply, a vapor barrier is to stop moisture in vapor state from passing THOUGH you wall / ceiling material by capulary action, or, for blacke of a better term, conduction. An air barrier stops both heat & moisture being carried by the air from convecting AROUND the wall & vapor barrier. THIS is the most important to stop! In an average heating season, if you take a 4 X 8 sheet of drywall w/o any vapor barrier, about 1/3qt of water will pass through it. If you take that same 4 X 8 sheet of drywall with a vapor barrier and have a 1" hole in it, 30qt of water passes through it, along with the heat in the air acting as a carrier.
Building codes address both issues, vapor barrier & air movement barrier (Today I especially know this, as I just received the 2001 revisions to the Wisconsin codes).
If you wish to see some areas that help with this info, try theses sites: www.energymatch.com - Check out the energy college www.weccusa.org