Ferrell gave an interview on the weekend suggesting that he will be joining the show full-time and not just for the final 4 episodes as reported:
http://insidetv.ew.com/2011/03/28/wi...ice-full-time/
Most people assume he was kidding, but, you never know. People have pointed out that Carell has been able to make a movie or two per year while doing the show, so it's not like Ferrell would have to stop making movies if he did take on the show full-time.