Started happening with me too, and I have ONLY used the OEM cable that came with the phone. I took it to a Sprint repair store and was told it was user damage and would cost me $120 to replace the phone.
I asked if this phone is designed to be charged.
The young woman I was dealing with asked what I meant by that.
I said that I plug it in once a day before I go to bed using the cord that came with the phone, so this is an obvious defect and should be covered under warranty.
Her reply was that it is physical damage and that isn't covered.
I said that I agree the damage is physical, but the damage occurred during normal use of the phone, and therefore should be covered under warranty.
She said I should have bought the insurance coverage and this wouldn't be an issue.
I said I shouldn't have to pay extra to make Sprint stand behind the products they sell, especially one that is only 4 months old and is still under warranty.
She said there was nothing she could do and gave my phone back.
As I turned to leave she said "Maybe you should call HTC since your phone is defective."
I turned back and said, "So you admit that this is a defect, and not damage that I caused?"
She realized she had screwed up and tried to take back that statement, but finally said she had seen too many come back for the problem for them all to be user abuse.
So again I asked why they would not honor the warranty.
She said they were instructed not to, but would not give me any more than that.
As I left I noticed everyone staring at me and only then realized the whole exchange had grown a bit heated. But, I was really pissed. I don't know what I'll do next. I don't really want to deal with HTC, as I have done that before and it was a nightmare. There is another repair center in town, so I may go try my luck there, since some people seem to have better luck than I did. For now, I can get it to charge if i mess with it, but it's a pain in the ass. It's too bad because I really love this phone, but it's been one problem after another since I got it.