My iPhone 4s has done the same thing over the weekend.
It was plugged in and displayed the charging symbol, but the battery % stayed the same overnight. The next day it regestered the charger, but did not charge and actually lost battery over time. Later that day it stopped acknowledging a charger out right.
I visited an Apple store and a 'Genius' looked into it for me. He told me "it is really weird...loads of people have been in recently with iPhone 4/4s that have started malfunctioning" - confirmed by this and the other threads.
They acknowledged my charger port was clean and undamaged "in great condition". They took it out back and opened it up, then came to report that there appeared to be absolutely nothing wrong with the phone internally. They did notice, however, that the water damage indicator inside, at the bottom of the headphone port had been activated. While I'm sure plenty claim this, I swear there is no way it could have water damage.
It would be interestng to know if anyone else's phone is displaying this?
I was told that my options were;
1) Pay c.£150 for a service - on a phone that when opened displayed no issues and which may not result in a working pgone afterwards....
or ( / then!)
2) Buy a new phone - convenient...
Reading the previous posts, taking into account the conincidental timing (being just over 2 years since the 4s was released, soon after the 5 has come out and a new software update) of these blatantly related issues and the suspicious appearance of a water damage indicator my conspirical bones are trembling......................
Apple?