im having the same problem...but it only happens when i plug the phone in when its off.
if i plug it in to charge and then turn it on, it gives me the unable to charge message, but if i plug it in when the phone is on, it seems to work fine.
also when its plugged in, the battery indicator shows a very low level (only one bar) but when i unplug it, it shows a full charge. why would this be?