I haven't heard that, but the only person who could really tell you is Mark Hamil.
I don't think I'd say he hasn't done anything significant... He's actually done A LOT of stuff on the voice acting side of things. Probably quite a bit that you'd recognize if you heard it, but didn't know was him. Right now, he's probably best known for doing the voice of arguably the best Joker in the Batman franchise (and yes, I do prefer Mark Hamil's Joker to Heath Ledger's).