In a recent tweet Mark Hamill clarified his statement about not voicing the Joker anymore:

Just thought I would clarify that my Joker comments are not related to #ArkhamCity

So worry not Joker fans, the Hamill is still here for your Clown Prince of Crime voice acting needs. Although, if he wasn’t referring to Arkham City what was he referring to then?

As a juicy tid bit Hamill tweeted this:

Thats a dream unfilled! Id love to be apart of that project! @NecromorphZ:What are the chances youll play the joker in killing joke

I just love how Mark Hamill voices the Joker. It’s how the Joker SHOULD be voiced. Any other Joker voices are lame. Just saying. 😉

[Source]