Based on some intriguing tweets, it looks like they may be casting the Dothraki people a little darker than most had expected. Of course, the books describe the Dothraki people as having “copper skin” and “dark, almond-shaped eyes.” With the casting of Momoa as Drogo, it appeared they would at least be keeping the copper skin aspect. But now it looks like they are willing to deviate from the book’s description of the Dothraki people and go in a more “multi-ethnic” direction.
Second tweet came from London-based actor Errol Clarke who didn’t state outright he was auditioning for Thrones but, given his mention of Morocco and Northern Ireland, it seems highly likely. (The America in his tweet probably refers to the fact that it is an HBO show.) Also, if we look at his actor bio we see he has martial arts and edged-weapons training, which is a strong indication that he is auditioning for one of the bloodriders.
Of course, adding to the idea of a multi-ethnic Dothraki people, is the fact that all of the Dothraki dancers from the wedding scene in the pilot are black.
All of the above evidence indicates to me that they will be portraying the Dothraki in the show a bit differently than how they were described in the books. Not as a race of strictly one color, but a multi-colored one.
[Props to Phoenix_torn for finding the Errol Clarke tweet.]