Listen.... Rogen is very good with Rowlf. Really he is. But not so good that he would abandon a lucrative spotlight career with films like Pineapple Express and that one with the security guard whos name escapes me at the moment (Serve and Protect, right?), etc. etc. to take a lower paying anonymous job as a puppeteer. there has never been a celebrity that ever fully performed a muppet... there was talk Christopher Reeve helped perform a couple background characters from time to time (or, in one episode) of the Muppet show. Anyone else was just a voice for the puppet. Ben Stiller the Elf, the voice of Rita Moreno in the recording of "There's nothing we Women can't be" and Robert DeNiro as a cabbage, a dog, and Elmo- all Sesame Street examples by the way... and Mel Brooks in VMX. That's about it.
Plus, I think bill has made great strides in the character for the Youtube video and LTS...