Great point and one that will drive the Dims nuts, since they are in no hurry to confirm Roberts. They consider her on the court until a new justice is confirmed. If this drags on past the court's fall session start date it will be interesting to see which side prevails and indeed whether O'Conner takes her seat.
I suspect that if the RATs try to make a federal case out of this conundrum, created by the brilliant O'Connor, she will get a phone call from the White House suggesting that she submit another letter.