I'm having this problem on AT&T G2 (D800). Yesterday I tried to put a hair dryer on the back for a while and started to charge until it was 100%, now it was on 26% of charge and plugged it but started to show the message again. Then I tried another shot with the hair dryer but didn't worked this time.
Also I tried to restart the phone but it never power back on. Does anyone have a solution? It's really dissapointing having a $400 paperweight.
BTW, ask for a replacement is not an option.