The tragedy in Japan is no laughing matter and to be sure that no jokes are made that might be seen to poke fun at the nuclear crisis in Japan after the earthquake and resulting tsunami, episodes of the Simpsons are being editing in some parts of Europe. Censors are removing references to any nuclear disasters or radiation from episodes and simply keeping some episodes from airing.
The Hollywood Reporter reports that Germany, Austria, and Switzerland are all editing episodes to remove any jokes or references to disasters at the nuclear power plant where Homer "works." A couple episodes of the show have already been pulled including episode 66 called "Marge Gets a Job" and Episode 346 "On a Clear Day I can See My Sister."
Those episodes have jokes about meltdowns and other things having to do with the nuclear plant. In total eight episodes are being held back until the end of April and then they will be reviewed again to see if the jokes are appropriate. I would assume once the Japan crisis with the reactors is squared away Europeans would again get normal episodes.