In the past they've gotten folks like Cloris Leachman, Judd Hirsch and Seth Rogan to do a brief (and kinda pointless) cameo in episodes here and there.
The more bewildering ones, though, are virtually "invisible" appearances where they do a random voice for (sometimes) a throwaway character. Example: Bryce Dallas Howard (from THE VILLAGE and SPIDER- MAN 3) does a voice in the Lauren Conrad episode.....in a cutaway.....and it's 1 line.
How the hell do they get these name actors for roles like this? I can understand it if they're fans and get a chance to be part of an episode. But could it be that they're they sought out by Seth MacFarlane & producers, and actually paid, to just do some dialogue for a background character?