First, Rowan Williams is not wrong for the reason that lots of conservative commentators within the Anglican Church who have been given airspace on Radio 4 (e.g. on Saturday's Today programme) say he is wrong: i.e. that he is challenging the Christian (or, a phrase I hate, "Judeo-Christian") roots of English law . English law is a hybrid, monstrous beast, with Protestant tradition being part of its story. If it is worth defending, or rather, those parts of it which are worth defending, don't have an awful lot to do with Christian tradition.
Second, Rowan Wiliams was wrong to use words like "inevitable", partly because it adds fuel to the growing paranoia of many people in Britain and elsewhere that the Muslim presence is some unstoppable juggernaut hell-bent on subordinating Britain to dhimmitude, which I still believe is an irrational position. Rowan Williams was not "opening up debate"; he was (presumably stupidly, unwittingly) stoking the flames of the culture wars.
Third, it seems clear that the majority of British Muslims think Rowan Williams was wrong. The Muslim Council of Britain, who adequately represent only themselves, reliably defend the old codger. But ordinary Muslims in the street interviewed on most news programmes think he's wrong. Tariq Ramadan, who, with all his strengthes and flaws (and he has many of both) probably does represent the mainstream of European Muslim opinion, has been sharply critical of Williams. More on Tariq Ramadan at the Flesh is Grass post mentioned above.
I am coming around to thinking that I was too hasty and knee-jerky in my response to the multiple marriage issue. What, actually, is so bad about multiple marriage? I might post properly on this, but in the meantime, comments welcome!