An Op-Ed piece by Kenan Malik that was published in the New York Times proposes that multi-culturalism, as it implemented in England, is a failure:

The real question is not how people like Mohammad Sidique Khan, the leader of the 7/7 bombers, came to be radicalized, but why so many young men, who by all accounts are intelligent, articulate and integrated, come to find this violent, reactionary ideology so attractive. To answer it, we need to look not at extremist preachers or university lecturers but also at public policy, and in particular the failed policy of multiculturalism.

The article focuses on the events of July 7, 2005 in London, England, when four terrorists detonated bombs in London’s transport system, killing almost 60 people. 

While not to minimize the tragedy and significance of that attack, it seems a little unfair to call multi-culturalism a failure over this event. Surely, there is a failure somewhere when home-born individuals  attack their own country based on foreign influences.  Whether this is a failure of multi-culturalism or something slightly different is debatable.

A full accounting of how people have integrated and have contributed to their society in positive ways over time – and how governmental policies have played a role in that – also needs to be considered.  Can an argument be made that official policies supporting multi-culturalism engenders immigrants – as a whole – to their nation and therefore acts in opposition to radicalizing influences? 

It seems to me to be a reasonable position.