@ just my tuppence
There is an amazing amount of woolly thinking on this subject, based on a fundamental confusion which also underlay the debate about the government's ill-fated "religious hatred" legislation.
It's actually quite simple: there is a difference between race and religion.
I make a very clear distinction between those qualities of people which are a matter of birth/genetics (race, gender, sexuality) and those which are a matter of choice, such as religious belief or political persuasion. It's not OK to attack people for what they are: what they believe is, however, fair game.
Once you get your head round this, you've answered the problem of where lines are drawn. Some muslims accuse westerners of double standards with respect to freedom of speech, using as an example western attitudes about anti-semitism as opposed to "Islamophobia". This is based on the confusion just mentioned, and in this context Islam is essentially a victim of its own success. Jews and Sikhs (in the UK at least) are protected by anti-racist legislation. Any percieved protection of Judaism and Sikhism is a side-effect of those religions pretty much coinciding with defined racial groupings. Islam, by contrast, doesn't map neatly onto one racial group: there are muslims in most such groupings. Judaism, Sikhism and Islam are in fact treated equally: all are beliefs, and therefore subject to criticism, mockery etc. The only religion in the UK with any special protection is Christianity, and for the most part that protection is ignored as archaic, only being dredged up by the likes of Mary Whitehouse (eg her homophobic persecution of that poem). In my opinion, those protections should be repealed: the answer isn't to apply blasphemy laws to all religions, otherwise you're protecting all sorts of half-baked cults etc.
Following the above through logically, and certain distinctions become clear, eg: criticising Israelis for using Judaism to justify the terrorist takeover of Palestine is fine. Making racist criticism of Jews as a people and denying history (eg the holocaust) is not.