My opinion on the matter is that it's fine. In fact I believe that save for those circumstances in which a character's whiteness is integral to their story, the casting calls for all caucasian characters from the comics being adapted from now on, should look at actors from all ethnicities.
I can understand the reluctance some people have towards it. You want to see your favourite characters on screen as they are in the comics. I get that, and I was thinking about how I would react if some of my own writing was one day adapted on screen and my characters where racebent. It would feel weird. However, I think there is a bigger picture than completely appeasing the core fanbase (who probably won't be entirely pleased anyway). Injecting a bit of diversity into the films is more important than that. And besides which you're never going to get an exact representation anyway. Most male actors don't have V-shaped torsos, most female actors aren't sporting E-cups. Where we draw the line between what is and isn't an accurate representation is totally arbitrary.
It's all well and good to say "let's just adapt the pre-existing ethnic characters on screen!" But if you actually look at those characters, it's a very small well to draw from. Here's Wikipedia's list of Black Superheroes:
http://en.wikipedia.org/wiki/List_of_black_superheroes#Marvel_Comics
The amount of actually major/semi-major characters there that Marvel has the rights to is pitiful. Seriously actually looking at that list shocked me. War Machine, Falcon, Luke Cage, Nick Fury, Black Panther, Blade, Miles Morales, Deathlok, Patriot, Misty Knight. That's really it.
I think the end goal should be getting more ethnic lead film franchises in the MCU. So let's look at that. Black Panther
is getting his own franchise. That's great. and Luke Cage is getting his own TV show. Blade could get a film series too. But that's it for the near future. War Machine, Fury and Falcon are already in the MCU (As prominent characters even if they aren't leads), as is Deathlok. War Machine is almost certainly dying in AoU, and Bucky seems to be the one who will take up the mantle of Captain America for the next several movies. Deathlok is a footnote. If Peter Parker is the MCU's Spiderman we probably won't be seeing Miles Morales as the lead for a over a decade. Misty Knight is probably more for SHIELD than the films. That leaves Patriot and I find it unlikely that they will introduce another potential future Captain America when we already have two. Not for many years.
You simply can't get any decent amount of representation by only casting those Superheroes who are minorities in the comics.