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 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.