I generally hope that when a candidate accepts a position, he or she will stick with it for at least a year. Sometimes there's a long learning curve, or it can take a while to really get to know others on the team well ... enough so that you really feel that you're a part of the team.

Having said that, unfortunately most companies will not affords employees the same consideration. While my hope would be that you'd stick with the commitment you made when you accepted the position, there's certainly no harm in keeping the lines of communication open with company A.