Walking to A from B is no big deal. Cross to the USAirways side of the building if you're not already there, go to the ground (arrival) level, turn left and follow your nose. The sidewalk takes you to the ground level of A in a few minutes.

I'd suggest walking if the weather is decent & you don't have much to carry, since the bus goes wrong way through the airport (it goes counterclockwise; you want to go clockwise) on top of waiting time for it to arrive.

Finally, BeantownFlyer has his/her terminals reversed. The USAirways shuttle was in B when I took it to LGA last Friday. Delta's seemed to be in A when I was there for Cape Air to Martha's Vineyard. I doubt they switched them over the weekend without telling anyone.