I remember watching Tim Russert try to tie Obama to Farrakhan, and thinking, "Don't they know this dude has been paraphrasing Malcolm X? Why aren't they asking him about that?" Not that I'm in favor of any of that, but I think this is what fueled so much of the "he's not really black talk"--most white reporters don't really know what black is. And so while they were waiting for Obama call for reparations or another Back To Africa movement, the missed the subtle things.
If White folks view Obama has "post-racial" and wonder if he is "really Black", it's mostly because they have a ridiculously caricatured idea of what Black people actually want or how they behave. Blacks themselves, of course, are far more steeped in the culture, and can recognize who is real and who is posing.