Actively supporting gay marriage legislation at a crucial moment certainly goes on the list of good things Dick Cheney has done in his life. But it does not begin to erase his lifetime deficit on even this single issue, since he (and Mehlman) were both participants in the Republican party's wildly cynical 2004 election strategy of putting anti-gay measures on ballots in key states to increase turnout among social conservative voters.
Minnesota Republicans are trying that move again, putting an amendment that would ban gay marriage in the state on the 2012 ballot. If Dick Cheney and Ken Mehlman work against that bill, even at the cost of whatever turnout advantage it might help Republicans gain, they'll start deserving real credit for a principled stance. If the only votes they'll work to sway are those of legislators in solidly Democratic states, though, they'll never make a dent in the negative balance they ran up in 2004.