Several articles about this have been posted in the News Thread (yeah, it's all proper and capitalized and I expect you to go there), but I figured this was deserving of its own thread.
Basically, the cast of the show has gone on strike. For two table readings, the following cast merabers have been absent without notice: Dan Castellaneta (Homer, Krusty, Grandpa Simpson, Mayor Quiraby), Nancy Cartwright (Bart, Nelson, Ralph Wiggum), Julie Kavner (Marge, Selma, Patty), Yeardley Smith (Lisa), Hank Azaria (Moe, Apu, Chief Wiggum), and Harry Shearer (Mr. Burns, Ned Flanders, Principal Skinner).
For more details, go on the thread. And for more discussion, um, start posting.