"The results surprised me. Apparently the optimal number of digits to include is 1, and the more digits you have the more likely it is to be registered. In fact, it appears that someone has registered every dot-com combination of 4 digits."
It shouldn't be completely surprising. The more characters (c) than digits (d) in a name, the more options available. 3c1d has more options (26x26x26x10=175760) than 4d (10x10x10x10=10000).
http://en.wikipedia.org/wiki/Factorial
EDIT: And also http://en.wikipedia.org/wiki/Probability_theory