If you reversed the roles, and it was a movie depicting the Zulu pantheon and the role of Nomkhumbulwane was played by a white actor - then I begin to see where the issue lies. Yes, the Norse Gods were traditionally depicted as white since the Norse people were white. But there's a lot more to it than that.
For one, there are plenty of roles for white actors in Hollywood. If it was a Zulu pantheon, you could argue that a white actor would be taking a role that would be much more suited for a black actor who may not have as many opportunities in Hollywood.
Secondly, they are fictional characters that don't exist and have never existed. It's not like a black guy was chosen for the role of Teddy Roosevelt. There is no 'accurate' racial portrayal of Heimdall from which to deviate, so it doesn't make any difference what race any of the make believe Gods are.
Thirdly, as was discussed in Penny Arcade - the entire movie is literally about a prototypical Aryan superhero. Let it have its one token black character, white people still come out on top by a pretty large margin.
Finally - they are Gods. Presumably they are Gods over the entire world and thus over all races of men. If the argument is "the Norse Gods were white", then the counter-argument is, quite simply, "how do you know that"? If we're arguing that there is an accurate way to portray a Norse God, then we're arguing that they exist and look a certain way. Since I've never seen Heimdall, I really don't know what his skin color would be.
Seriously though, I hope this movie doesn't suck. That trailer looked sweeeeet.
