Does it charge if you connect to your computer? Are you using the same cable with all of the chargers? Is there any visible fraying on the cable insulation?
Hmm, I'd suspect an issue with the dock connector on the iPhone in that case. Bring it to an Apple Store and if you're in warranty they shouldn't give you any grief over replacing it outright.
The AppleCare on an iPhone is good for 2 years, so you should still be under the coverage it provides.