Here's President Obama stumping for Barbara Boxer and interrupted by hecklers demanding the repeal of "Don't Ask, Don't Tell." Obama turns on some charm and says "We are going to do that," and the crowd starts chanting "Yes, we can."
Now, I interpret the "Yes, we can" chant as sarcastic. You say you "are going to" do something, but why haven't you done it? You ran for office telling us "yes, we can," and you got the power of the office, so you've had time to demonstrate your vaunted capacity to do things. To tell us now that you will do something in the future isn't good enough. Sarcastic, right? If not, it should be.