While we're discussing the ludicrous concept of "defamation of religion", I'm wondering why no one talks about defamation of science. Maybe we could try to ban Mary Shelley's Frankenstein and all those other books, movies, TV shows, etc., that present stereotyped images of irresponsible, hubristic scientists. Maybe we could ban a film like Expelled, with its slanderous claim that genuine biology in the form of Intelligent Design is being kept out of the academy by a conspiracy of Darwinists ... not to mention the claim that Darwinian evolutionary theory was somehow to blame for the Nazi Holocaust. Then we have the relentless war on science conducted by British journalist Bryan Appleyard, who believes that Nazism was not merely a misuse of modern science - with its mechanised attempts to exterminate the Jewish people - but somehow exemplary of it.
In fact, once we start looking into it, we can see that the defamation of science goes on and on. If the idea of defamation - a legal category to protect individuals from being shunned, or socially ostracised, by attacks on their reputations - scales up to include contentious debates about ideas and organisations, then let's get on the case. It seems that science has a particularly strong claim to be protected against defamation in this extended sense.
Of course, once this example is used, it becomes clear how ludicrous the idea is. Defamation law is not fundamentally about protecting organisations and it is certainly not about protecting ideas. When ideas are criticised, the correct response is not to march off to the courts to obtain an official ruling on a claim such as "evolutionary theory contributed to the Holocaust" or "the Koran contains material that lends itself to support of terrorism". Ideas should be met with ideas. Criticism of ideas should be met with counter-criticisms.
Likewise, large organisations can take care of themselves in public debate. There might be a need for some kind of law to protect share markets from instability caused by false rumours, but that purpose would be remote from the purpose of defamation law. Generally speaking, it should not be possible to sue for defamation of a company, as opposed to individuals connected with it. Here, the existing law should be scaled back if anything. It certainly should not be possible to defame something like a church or a religious sect. Once we talk about a religion itself, not its individual members or the organisations they have formed, any analogy with defamation has become so tenuous as to be laughable.
Outside of very narrow areas that should not be expanded by dubious analogies, the answer to bad speech is better speech.