I only just saw it today, in an offhand remark in a comment on Pharyngula. But I'd never heard it before, and I love it.
It's the atheist version of Pascal's Wager.
In case you've never heard of it, Pascal's Wager is an argument for believing in God that goes roughly like this: If God doesn't exist, nothing very bad will happen to you if you bet wrong and believe that he does. But something very bad will happen to you -- i.e., you'll go to Hell -- if God does exist and you bet wrong and believe that he doesn't. Therefore, believing in God is a better bet.
Now, there are a million things wrong with this argument. The most obvious ones are: (1) how do you know which God to believe in, and which of the zillion existing religious practices he wants you to follow?; (2) a Pascal's Wager belief isn't a sincere belief, and if God does exist he isn't going to buy it; and (3) many people, myself included, strongly disagree with the assertion that nothing bad will happen if you believe in a non-existent God. (Despite it being a crappy argument, assorted versions of Pascal's Wager still get used again and again by many religious believers; hence the discussion on Pharyngula.)
But this is just excellent: It's the atheist's version of Pascal's Wager.
Here's how Tatarize (of God Snot, Where God's Not) put it in the Pharyngula comment: "Do good, then if there's a evil God everybody is still screwed. If there is a good God then you go to heaven, if there is no God then doing good is its own reward."
And here's how I'd put it, slightly fleshed out:
Be a good person, by your own good faith beliefs and efforts, and don't bother with what God thinks.
If God is a capricious evil bastard who keeps changing the rules and doesn't play by them anyway, it doesn't matter what you do. You have no way of knowing what he'll reward or punish. So you might as well just do what you think is right, regardless of what God may or may not think.
If God is good, by any reasonable definition of the word "good" that we can comprehend, then he'll reward you for being a good person regardless of whether you believe in him or not.
And if God doesn't exist, there are still plenty of reasons to be a good person: feeling empathy for people and a sense of connection with them, yada yada yada.
I don't know why this tickles me so much. I certainly think there are better, more serious arguments for atheism than this one. But Pascal's Wager is just such a millstone around our necks, and I think this is an unusually witty and clever response to it. I've always thought that fear of God and hell was a terrible reason, ethically speaking, to be good; a child's reason, really, "If you hit your sister you'll get sent to your room." The religious believers that I like and respect aren't good people because they're afraid of hell. They're good people for good reasons, grown-up reasons, "empathy and connection" type reasons. And regardless of whether you're a believer or not. I think the Atheist's Wager points up beautifully the absurdity and childishness of the fear-based, "hedging your bets" version of beliefs and ethics... and the ethical strength of the grown-up version.