As I mentioned below, the debate on multiculturalism is so broad that it seems to mean almost anything to anyone. However, I've heard a few people confuse the terms race and culture.
I heard one commentator on the radio claim that Britain's model of multiculturalism was more successful than America's. As evidence he cited the greater number of inter-racial couples you see in Britain than you see in the US. I'm sure he's right about the numbers of inter-racial couples, but I don't know what that has to do with multiculturalism.
The problem the US had (and still has, but to a lesser extent) with race was that one group of people was purposely excluded from the common culture. 'Separate, but equal' was the view. Black people weren't seeking multiculturalism, they were seeking a place in the common culture.