What Obama did say was “That’s not true.”, and that was servicable and dignified.
In hindsight: " Well, I say it ain’t so, Joe", with a big ol’ wink.
“Like a rug…”
I like this one.
I can give you Obama’s likely response:
“Please be nice to me. Please”
LOL. That was my first though, I went on youtube to look for that video and couldn’t find it.
“Who let Glenn Beck in here?”
Rush, you’ve lost weight! But from your behavior I can only assume you’re still poppin’ the pills.
“Well, your mother does more than just lie, that’s for sure.”
“Did you just call me a nigger, Joe?!”
Let THAT bleed out into the media and see if he gets re-elected.
“Takes one to know one”
“I will sign a statement tonight promising to resign if even one illegal immigrant gets coverage in any health plan I endorse. Likewise I will resign if I endorse any plan that grants amnesty to even a single illegal immigrant.”
That would show him!
“The wife’s fine but the kid’s a retard” 
Didn’t Obama actually say “That’s not true” before he continued?
I suggest “Boing fwip!”
“Shut the fuck up, you ignorant inbred saucer of goat semen.”
“I’m sorry - did you say something? No? Didn’t think so.”
Rodney Marsh apparently said this to Ian Botham during an Ashes match. Botham is supposed to have replied “Wife’s fine. Kids are retarded”
ETA: Sorry, Malacandra!
Sorry, no pie.
I think the best response was saying nothing.
To be real classy, he could have turned it into an Epimenides Paradox by saying, ‘Suppose I say that’s true. Does that mean I’m lying, or not?’.
I think that would have crunched a few of Wilson’s mental gears and winded him.
A truly devastating way to handle this, known to many public speakers, would have been for Obama to make it clear he was temporarily abandoning his own speech to allow for the interruption, and in a calm, dignified manner turn to Wilson and say, ‘Although this isn’t really the time or place, you have just accused me of lying and obviously feel strongly about it. So, let’s see. Please share with us your carefully reasoned objections and your well-worded case supporting your contention, demonstrating that you have your facts straight and some substance to offer, not just inane, random exclamations’.
Then wait with a smile of benign, patient tolerance.
“And you sir, are a republican congressman.”
“All right yes: Santa isn’t real. I’m sorry I suggested otherwise in front of your kids last Christmas.”
“…I’m not certain the gentleman realises the difference between lies and truth.”
“So it speaks! I am most impressed. Now that was a lie. Go ahead, take notes.”
"Yeah? Well, you’re ugly and your mother dresses you funny. BOO-YAH! Suck on that!
To continue … "