Firstly, english is not my native language, so this might be a little off..

I'm norwegian, and I've always been fascinated by the norse mythology (not believing mind you, I just like the stories). And while I have noe problems with a norse god being portrayed by a black man, I find it funny that they chose a black man to play Heimdall.
The following is from wikipedia in norwegian: "Navnet Heimdall betyr kanskje «den klartlysende» eller «den som lyser over verden». Dette passer godt med kallenavnet "den hvite ås", og at han omtales som «den hviteste av alle æsene»."
This says that name of Heimdall is meant to be interpreted as "The one who shines over the world".. so no problem so far.. the problem comes when we reach his nickname: "the white hill". And also the fact that he's described as "the whitest of all æsene (read: norse gods).
Still I look forward to seeing the movie, and I'm sure Idris will do a good job in the movie.
Signing off...