Donald Draper wrote:
I have had the very same thought. He was emotionally and psychologically crippled. It took a real injury at the hands of Bane to over come all that.
The limp could also be the result of the fall he takes at the end of TDK. That same fall kills Dent but its likely that he landed badly and really hurt himself.
That's what I assumed. He and Dent take a nasty tumble. Afterwards, Batman manages to climb onto his bike and zoom off, but that's the last we see of him as Batman, so it's very possible his knee got wiped out there.