I just have to say it was totally worth it to build a new character from scratch on DC Universe Online just to hear Mark Hamill reprise his role as Joker. Unfortunately his last voice acting Joker role was for Batman: Arkham City.
By the way, most people don't know it but Mark Hamill is a MAJOR player on the voice acting scene. Take a look at his profile (which I thoughtfully provided earlier). I should probably put Arkham Asylum and Arkham City on my to-play list.