I took one a couple election cycles ago here in Utah for a mayoral election. This year, with so many early voting, I think exit polls are also going to include live telephone interviews to get a better read on who's voted.

As for calling the winner, the networks will call if the exit poll results are outside the margin of error. If the exit polls show Obama up, say, 1 in Florida, it's unlikely they'll make a call until the raw vote total comes in and at the very end, when it becomes apparent Romney can't win the state with how many votes remain not counted, they'll finally call a state.