Everyone (including Joe Biden, who also thinks he's "clean") says that Barack Obama is articulate. I've never seen any evidence of it, and there was apparently plenty of counterevidence at the Rick Warren thing. Being good at reading a teleprompter is not the same thing as being "articulate."
Depends on what you mean by "articulate". If you mean Obama speaks clearly, as in "Obama articulates his words well", then I think he does. If you mean Obama explains his ideas clearly, as in "Obama articulates his thoughts well", then I would say that is different from reading a teleprompter.
Didn't see the debate, so I can't speak about it.