mathematical physics - How does the sum of the series “1 + 2 + 3 + 4 + 5 + 6…” to infinity = “-1/12”?

How does the sum of the series “1 + 2 + 3 + 4 + 5 + 6…” to infinity = “-1/12”, in the context of physics?

I heard Lawrence Krauss say this once during a debate with Hamza Tzortzis (http://youtu.be/uSwJuOPG4FI). I found a transcript of another debate between Krauss and William Lane Craig which has the same sum. Here is the paragraph in full:

Let’s go to some of the things Dr. Craig talked about. In fact, the existence of infinity, which he talked about which is self-contradictory, is not self-contradictory at all. Mathematicians know precisely how to deal with infinity; so do physicists. We rely on infinities. In fact, there’s a field of mathematics called “Complex Variables” which is the basis of much of modern physics, from electro-magnetism to quantum mechanics and beyond, where in fact we learn to deal with infinity; without the infinities we couldn’t do the physics. We know how to sum infinite series because we can do complex analysis. Mathematicians have taught us how. It’s strange and very unappetizing, and in fact you can sum things that look ridiculous. For example, if you sum the series, “1 + 2 + 3 + 4 + 5 + 6…” to infinity, what’s the answer? “-1/12.” You don’t like it? Too bad! The mathematics is consistent if we assign that. The world is the way it is whether we like it or not.

-- Lawrence Krauss, debating William Lane Craig, March 30, 2011

Source: http://www.reasonablefaith.org/the-craig-krauss-debate-at-north-carolina-state-university

CROSS POST: I'm not sure if I should post this in mathematics or physics, so I posted it in both. Cross post: https://math.stackexchange.com/questions/630490/how-does-the-sum-of-the-series-1-2-3-4-5-6-ldots-to-infinity

EDIT: I did not mean to begin a debate on why Krauss said this. I only wished to understand this interesting math. He was likely trying to showcase Craig's lack of understanding of mathematics or logic or physics or something. Whatever his purpose can be determined from the context of the full script that I linked to above. Anyone who is interested, please do. Please do not judge him out of context. Since I have watched one of these debates, I understand the context and do not hold the lack of a full breakdown as being ignorant. Keep in mind the debate I heard this in was different from the debate above.