Confession....You raise a rather interesting point, that in the case of Obama, the potentially offensive discouse about blacks sounding articulate (by the standard of not "sounding black") may indeed run up against a person who truly is articulate by any standard. In fact, rather than highlighting Obama's racial status, such language could be intended to contrast him against certain inarticulate recent American presidents (as David Letterman's "Great Speechs in Presidential History" repeatedly underscores), a point in which race is not foregrounded. But a person intending to make this point may easily be misunderstood (especially since all the current candidates are more articulate than Bush), as there is already a broader discourse in American society about articulate blacks, which reflects the tendency that one's intended meaning is not necessarily communicated in a discursive context that imposes a more familiar social meaning. Anyway, we shall see tonight how Obama performs under the hot spotlight of a presidential debate.
AK-Jeff...Indeed, the dialect of English that is commonly perceived as "sounding black" (which, as a sociolect, originated in the U.S. South in black communities under segregation), is very closely related to certain varieties of southern white English, sharing many features in common (such as the pin/pen merger, in which /eh/ sounds like /ih/ before nasal consonants, the monophthongization of diphthongs like /wa:t/ as "white", the deletion of liquids before mid front vowels like "he'p" for "help", the use of "aks" as a variant for "ask," the use of done as a completive verb, etc.) -- revealing the common origin of both. I actually taught a class last year that covered this subject. Many of these features, in turn, come from certain British dialects (especially Scots English and northern dialects), and there was a much higher proportion of Scots speakers in the U.S. south than in the north, where other British dialects were much more of an influence. So black families who always lived in the northern states since the colonial era would have spoken a different dialect than the blacks who originally came from the south (in the Great Migration). There is not one kind of dialect spoken by American blacks, the only reason why this one dialect is so common that whites generally think of it as "sounding black" is that the majority of blacks who spread throughout the U.S. during the eras of reconstruction and Jim Crow came from the southern states which had developed a more distinctive way of speaking compared to the dialects of the northern states. If you go to London, you'll hear a different dialect spoken by blacks over there, whether Cockney, Estuary, or some other British dialect there (or Jamaican or whatever).