According to several reports a small number of users are having problems with their iPhone power adapters permanently failing. Testers have tried replacing the iPod cable, different wall outlets and all of the standard things one would try to correct the problem, but the discussion thread on Apple’s support website indicates that several people are having the problem.
Personally, I have had no issue with my Power Adapter.
Still, if you’re having a problem, contact your local Apple store or call Apple support and Apple will replace the adapter as part of the iPhone hardware warranty.