Should Rush Be Sued For Defamation?

Usually Rush Limbaugh enjoys being in the news, but one gets the impression his recent appearances are not enjoyable for him.  Limbaugh has made news because of his calling a Georgetown law student a slut and a prostitute.  This type of news has made his sponsors nervous; at least seven have withdrawn their commercial support of Rush's talk show, but it is unknown for how long they will continue to do so.  It's likely that if the student brought suit against Rush for defamation, sponsors would not quickly resume advertising on Limbaugh's program.  Such a suit might also serve to temper the crazy talk of the wild men of the right and left.