Go to the Apple support site. There is a warranty page that will ask you some questions (phone serial number, etc) and will ask you what the problem is you are experiencing. They will probably send you a new cable at no cost. You may have to return the defective cable to Apple, but I'm not sure.
I did this when the wire covering seperated on a pair of iPod headphones. They sent me a new pair with no problems and did not request back the original set.
It's worth a try anyhow!