Warren Buffet voting for higher taxes isn't what is generally meant by "voting against ones own interests". Could be wrong, but in my mind the expression is about people who apparently vote against their own "stated" interests or goals. People who deride "big government" and then vote for people who enlarge it. People who complain about the loss of manufacturing jobs to NAFTA, then vote for the guys who passed it. And all of our favorites, the people who rally against "government run health care" all the while screaming "don't touch my medicare".
I agree that there are people with priorities that allow them to vote for people which apparently opppose SOME of their stated goals. It is a big reason for the "culture wars" to begin with. Reagan peeled off union democrats with "Guns, God, and Gays" and then trashed the unions. You see Santorum cranking up the culture wars because the economy isn't giving him anything to campaign upon. I do wonder what the population of the "stupid" people are that vote "against themselves" without actually understanding that they are doing it. Small percentages move elections, but I suspect that is all that it is, small percentages.