I think both camps are correct - it's based on contrast. My nephew, who happens to be bi-racial, like Obama is often told during job interviews that he "speaks so well!". He takes that comment as a shock that he isn't using Ebonics. The expectation that an African American might speak differently exists because people have experienced people of color who do speak in a distinct manner - the way all subcultures do. So the contrast surprises them.
In the case of Obama; the contrast between the countrified, colloquial manner in which Bush speaks (cowboy talk) and the precise and eloquent manner in which Obama speaks(a Harvard man)is noticeable to many people - providing a surprising contrast and sense of relief actually. Bush's style of speaking may have caused people to misunderestimate his intellect. I think this is due, in part, to "not being a reader."
