Actually, one of the best "excuses" I've read (posted by either South or jhpm at some point) for why Obama hasn't been impeached is exactly because he's black. Some retired general, who said there is irrefutable evidence that Obama is not a US Citizen, claimed that there will be no impeachment because of the race riots that would occur if a black President were impeached.
That's a great one because it has race baiting and the good ol' Republican "I have evidence but I'm not showing you it" excuse all rolled into one. A veritable souffle of bullsheep.