I found this thread very informative and would like to add the following:-
I could be wrong but I have often read in history books that mulatto comes from the word mule because (and in the language of the slavery period) in the early phase it was believe that if you were to breed black and white you would get offspring that cannot reproduce (as in crossing a horse and donkey to get a mule). While the term is still very widely used in South America I believe in reality it's more pejorative than positive.
I believe that people should be allowed to be exactly what they are or want to be. I agree that Kelly Holmes for example should not be listed as black as she is not - she is mixed. In addition she was mainly raised by her mum (white) and so exposed more to a white rather than black culture.
Why should we care about this stuff? Well perhaps we shouldn't but we live in the world that we have and not the one that we would like. In this world we have global media which currently says that we have to conform to certain aesthetic norms in order to be considered beautiful/attractive (whatever). This says that black women have to be light skinned and have long straight hair to be attractive ..and very few are or have. Having mixed race described as black confuses matter.
Lets take a little trip down tangent avenue to emphasise the situation/matters:
In Brazil blacks are at the bottom of the barrel - and to have black features is considered most unattractive. The only group considered lower are the indigenous Indians ? sometimes simply referred to as indigenous in the country. Recently money was set aside to develop indigenous (Indian) communities and overnight hundreds of new "indigenous" communities appeared. The Indians have been flattered that so many people all of a sudden want to be "indigenous"; the problem is that the scant resources that have been set aside are now being drawn down by other communities.
Coming back to our subject - I remember a time when no one wanted to be black. Then moves were made to provide resources and processes to help the black community come up to speed; now almost everyone who is not white seem to call themselves black.
Now everyone can say how egalitarian a society we live in because so many blacks can be seen on television and in public life when in fact most are not black at all but mixed (or other).
In addition the ones seen (on the box and in public life) tend to conform to the light skinned long (usually straight - achieved through extensions or chemicals) haired stereotype.
I recently took my youngest daughter (9) to a black hairdressers where she decided she wanted hair extensions. I was none too pleased with even the suggestion but the hairdresser tried to be convincing of how this was a good idea and quote "even white women are doing it".
I don't need my children to grow up radicals but I would like them to grow up with positive self images, and not feel ugly or inadequate because they have black features. I don't want them to have to make the compromise (or take broken self esteem remedies) to conform to norms to get the opportunities that hard work and merit alone should afford them.
So whether you are black, white, mixed or polka-dot should be totally irrelevant - but we live in the world we are in with the problems that we have. Our society have all sorts of problems - race and colour is only one. Fixing social problems start with debate - I believe this is why the state of being black, white or mixed and how the groups relate to each other is (currently) important. I think that to classify mixed race people as black hides (or at the very least confuses) a very serious problem.