I wouldn't disagree that this is a complex, and emotive, issue, but is it not curious that, if 'writers, directors, theatre companies, CDs, organisations, etc' are so aware prejudice is a problem they are beating their chests about it, yet doing nothing to rectify the situation? I actually cannot believe that the situation is quite so atrocious as some of these posts are making out, because we are no longer living in a society where all of the casting directors, writers, directors etc. are themselves members of a WASP male establishment. There are Black casting directors, Asian casting directors, Irish casting directors, plenty of Jewish casting directors and they are among the most influential people in our entire industry. There are increasingly strong moves (at the top tier of the industry, at least) to cast 'colour blind' work. There are actors of 'ethnicity' being given lead roles in prime time television. All of this marks a lot of progress in this industry compared to where it was in e.g. the 1950's, when it was riddled with racism, anti-semitism, Imperial elitism and so on.
Of course, the endemic limitations of the industry continue to prevail: Equity has a standing set of Equalities Committees for a reason, and they exist in order to ensure that equal opportunities law is upheld, the maximum number of opportunities for 'ethnic' casting are made available, and any serious infraction of race relations, defamation etc. is severely dealt with when it occurs. The system isn't perfect, and nor can it be totally policed.
Dan's point also has its validity, which is really to say, why is casting so automatically assumptionist, and e.g. more Ayckbourn plays are *not* cast with e.g. a Black cast? To which the only answer must be: because the casting director/producer/director etc. lacks imagination and/or wishes to honour the original conception of the work etc. etc. etc.
Yet, we do progress in relation to these issues decade by decade. In the 1960's, it was still very uncommon for ethnic actors to *be* working in the UK profession, and there were still prevalent practices of making up white actors to play 'ethnic' parts and the writing of scripts parading actively racist assumptions. By the 70's, we had progressed to inclusion of actual Black, Asian etc. actors, Jews were allowed to play Jews and so on, but the scripts were still often deeply offensive, often casting the actors as stereotypes, figures of fun and/or villains. By the 80's and 90's, the scripts had become broadly less offensive, but actors of 'ethnicity' were still being relegated to minor roles. We have now started, in the 21st century, to allow leading roles, romantic subplots, inter-racial relationships, and so on to be taken on by ethnic actors in prime time television, on stage etc. etc. There are still, perhaps, too few of these roles offered in comparison to those offered to a White British majority, however. The next stage of development may indeed see more movement towards what Dan suggests: an abandonment of the feeling that plays have to be circumscribed by the notions of the era in which they were written and can be performed by anyone, of any race or creed: there are already many leading stage companies that *do* pursue work in this way. Screen work may be harder to 'democratise' in this manner, because versimiltude is more vital to it - but, in point of fact, the world of screen is already considerably more integrationist than the world of stage.
Naturally, these stages of development have paralled the nature of the culture as a whole: it is easy to forget what a shock to the system of the White majority it was in e.g. the 1950's and early 1960's when the first major influxes of immigrants from the colonies reached British shores. It is easy to forget how blatantly anti-semitic Britain was in the days of Moseley, and post World War II. It is easy to forget how much resentment was felt about the Irish presence in Britain three decades ago. The stage and screen profession paraded and paralled wider attitudes. Today, we live in a much better integrated, more tolerant, pluralist society than was once the case, and this should be reflected better in our industry, perhaps. But I am still convinced that, however uphill the struggle may seem at times, we are still far, far better off in these respects than we have ever been.