I also had same problem after placing my iPhone in the cup holder of my minivan without knowing there had been apple juice spilled inside!!!!
I cleaned the dock connector with credit card. Didn't work. With alcohol and toothbrush. Didn't work. Then I noticed some blackish stuff in the grooves of the dock connector so I slid a toothpick up and down the surface a few times in each groove. That seemed to do the trick. Probably sum dried up apple juice that tricked the iPhone to thinking it was docked. Thanks all for the other posts!!!
Please note. The toothpick did crack so be careful not to break it off inside.