My Nike+ sensor was failing on me and recording times that were off, my pace was being recorded all over the place. I found out that the Nike+ sensor has a one year warranty. I visited the site http://www.apple.com/support/nikeplus/ and filled out the information for a replacement sensor. You do need to supply a credit card number, but your card is charged only if you don’t return the faulty sensor within 15 days of receiving the new sensor.
I received my replacement sensor within a couple days and it was shipped next day. The new sensor worked great and I returned the faulty sensor in the prepaid box that the new sensor was delivered in.
Overall, the replacement process went very smoothly and was very quick. Kudos to Nike for a fast turn around on the replacement and making the whole process very easy.