The New York Times | Play It Straight

"The best spin for the Democrats is that they're using Mary Cheney to paint their opponents as hypocrites: Republicans feign acceptance of all people, trot their own daughters out to demonstrate their fairness, then propose that the Constitution be amended to deny equal rights to same-sex couples. They want to have it both ways.
...
[M]aybe John Kerry or John Edwards would have stated the obvious point about the vice president: he doesn't support equal rights even for his own daughter."

Rubenstein gives an argument very close to my own.