At the beginning of the episode, there was a disclaimer stating that "all celebrity voices are impersonated".
The Simpsons has one of these as well on occasion (but buried at the end of the credits - occasionally it will also list who did a specific celebrity voice; I have seen "Jay Mohr as Christopher Walken" and "Jerry Seinfeld impersonation by Dan Castellaneta"). I wonder if certain celebrities pretty much demand this, or if the producers just feel that they need to do this for certain celebrities.
Note that Pat Sajak has voiced himself in cartoons - in the early 1990s, he voiced himself in an episode of Rugrats (he was the celebrity face of a sweepstakes, similar to Ed McMahon and American Family Publishers). This is not to be confused with Alex Trebek's role on the same show as a game show host, although his character's name there was "Alan Quebec" (he would later voice himself on a Simpsons episode).
