"The crux of Isbell’s Snake Detection Theory focuses on vision and is straightforward: 'Visual systems are more developed in those primates that have shared the longest evolutionary time with venomous snakes and least developed in those primates that have had no exposure at all to venomous snakes'."
There's also a very interesting and much longer review I just submitted: http://news.ycombinator.com/item?id=1195869
Edited: I see a lot of comments about snakes not preying on humans. But humans evolved over millions of years from much smaller primates. That's where Isbell thinks the evolutionary arms race took place. You could argue the reason snakes don't prey on humans is because our ancestors won that arms race.