Tami from the blog What Tami Said analyzes the racist implications behind the expectation that prominent African Americans (like Barack Obama and Oprah Winfrey) drop their "blaccents" and just "speak regular":
Lately, I've heard several blacks in the public eye taken to task for what some in the mainstream view as nefarious use of a black accent, or the cynical unleashing of a "hidden" black accent when among other African Americans.
There is absolutely nothing wrong with having a black accent, except that in a society where white is right and all other is wrong, a black accent is judged as less desirable. Making a call without your "white" voice on could mean the loss of a job, an apartment, any number of opportunities. So, as a matter of survival, upwardly mobile blacks learn to effortlessly code switch, that is unconsciously modify speech to slip from one culture to another. We generally reserve speech with ethnic markers for conversations with other people of our ethnicity.
Go give it a read. In her post, she also references the excellent book Shifting: The Double Lives of Black Women in America, which I totally recommend.