Like most Republicans on this posting, I agree it's ridiculous to feel guilty for slavery which occurred and ended a long long time ago.
So let's take look at what's happened recently.
Since 1968 the Republicans have consistently used the "Southern Strategy" to its advantage. That was a strategy of using code words and positions such as "States rights" and "Law & Order" to appeal to the Jim Crow crowd and to lock up the electoral collage votes in that region.
Republicans have not only benefitted from a fear of black people, but have openly stoked the flames, a la Willie Horton .
What's too funny it that many posters on this site claim that conservatives don't feel guilt because they are are beyond race. This is a bit rich. The Republicans have played racial identity politics it like a violin for the last 40 years. And continue to do so as we speak. Consider the recent La and Miss congressional races.
So along comes Obama. A thoughful, intelligent, well spoken individual who happens to be black. He's not claiming to be a victim and doesn't want a sympathy vote. He doesn't fit comfortably into the "angry black man" cartoon that has so greatly benefitted the Republican party in the recent past.
Hence it's only natural that some people would feel some satisfaction in voting for this fellow. Call it liberal pride rather than liberal guilt.