I can give you an official answer on this based on recent dealings with Nike.
I had a pair of Nike Air Max Breathe Free II in green and black that I sent in around March under the outsole warranty. I then purchased a pair of the French Blue Breathe Free II's before sending the green ones back. I received a voucher for the green ones. I then found a defect in the blue ones in which the plastic hexagon on the bottom of the shoe that connects the forefoot and heel cracked. I used my voucher from the green shoes for a pair of the chlorine blue cages and then sent the blue ones back and received a voucher on those. I then bought a pair of Breathe Free IIIs for $39. AS I alternated the IIIs and the Cages, I had the same cracking problem with the cages as I did with the IIs. I then sent the Cages in as a defective shoe and my voucher was restored to the $128 value. I discussed the warranty situation with Nike and the voucher is their third course of action. First, they try to send you the original shoes (but they do not stock the IIs on the web). Next, they try to send you a shoe with a very similar fit, but if they do cannot do that, they will (#3) send you a voucher, which is, in the Nike representative's words, "as good as cash." Take that to mean what you want, but I think a voucher is as good as buying the shoes new for the first time. So i now have two vouchers sitting at my house and a pair of $39 Breathe IIIs that will soon be sent in for another.