To use another Shakespearean reference, in Midsummer's it is clearly written that Helena is tall ("painted Maypole") and Hermia dark ("tawny tartar" and "ethiope") and while often that is interpreted as her simply being brunette rather than blonde... there's a case to be made - a strong one - for casting her with an "actress of color"... but color-blind casting, switching the ethnic backgrounds of the actresses simply because the white chick did better reading for Hermia and the black chick gave a better audition for Helena is foolish and would require rewrites for the casting choices to even make sense...Now, if you've got permission to rewrite the script, or if it's open domain, or the character is an ambiguous blank, by all means, cast color-blind. But here are a few anecdotes of my own as to why color-blind and gender-blind casting simply doesn't work. It's hard enough to suspend disbelief to watch a play; it's even harder when what you're watching distracts or takes away from the atmosphere of the play.
Take for example the current production of You Can't Take It With You at T. Schrieber Studio. Donald and Rheba are black, and they're most definitely the serving class. How ghastly. Except this is a play written in the '30s (as a film, it won the Academy Award in '38), and the current production is of a period piece, meant to take in--flaws and all--the situation back then. I was fine with Peter Aguero as Donald through the entire play; he carried himself with a portly bluff that made me guffaw many a time. Up until Rheba (Shirine Babb, who is a black actress) remarks, "I sure am glad I'm colored." To which Aguero, who is a white actor (and sketch comic), replies "I sure am too." Laughter, but not at all for the right reasons.
In an old college production of mine, Rosencrantz and Guildenstern Are Dead, our director chose to make the Player a woman. If the actress had been playing it as a man in drag (like most of the Player's troupe), the jokes would still have worked, but to translate the role itself just makes the jokes fall flat. Not that a woman being the pimped-out ringleader of a bunch of male "actors" is a bad idea, but it doesn't fit with Stoppard's jokes, and it devalues the role of Alfred, the most girlish lad, and the one who gets the most "parts."
It also puts a double standard on a woman who's playing the role as a man, as she has to work twice as hard to play the role as a guy, and not as a woman dressed as a guy: another production I worked on (Picasso at the Lapin Agile) had both Elvis and Gaston played by women. The girl playing Elvis was phenomenal, and the director actually used the genre to make some underlaying jokes (of the double entendre kind). The one playing Gaston, on the other hand, was just flat, because the one thing that needed to be true -- that it was old, lecherous man -- was constantly undercut by the fact that it was obviously a young (albeit lecherous) girl.
I can't speak to much of the rest of the debate about color on stage -- I don't know if there's a reverse racism (or not, as Matt Freeman asserts), but the noble cry of any role being open for any actor just doesn't wash with me. And I wouldn't want to whitewash it either. There are plenty of plays out there for women and black actors -- plenty of good ones, too -- and while I'd kill to be in Topdog/Underdog, if a really white guy acting really black would take away from the show (or subvert the point of the play), you have to wonder if you're not just making a different kind of art at that point. That, I'm all for: I'm against copyrights when they stand in the way of just making a good performance. But let's not call Raisin in the Sun that if it's got an all-white cast (although there are white raisins); at that point, it's not Hansberry's play.
As for the examples being given of black actors playing "white" roles in Shakespeare -- aside from the fact that Shakespeare is timeless and part-fantasy (even his history plays), most companies have already changed the way his plays were done, and I don't think Liev playing Othello or Denzel playing Macbeth would really change that much.