True. Both effects might true. However the 2 don't cancel each other out probabilitisticly. Given that McCain is so far behind the mere possibility of a Bradley effect favours him. Let's say both effects are 50% likely to exist and would sway votes by enough to tip the election. Then the results would be

25% Bradley + No Cellphone = McCain wins 50% Bradley + Cellphone = Obama wins 25% No Bradley + Callphone = Obama wins

So in this example McCain now has a 25% chance. Of course this is an exagerated example but hopefully you get the point.