Sam Arbesman and Greg Laughlin, the authors of the scientific paper, used the history and dates of previously discovered exoplanets to develop their system. What they did was:
Using the properties of previously discovered exoplanets, we developed a simple metric of habitability for each planet that uses its mass and temperature to rate it on a scale of 0 to 1, where 1 is Earth-like, and 0 is so very not Earth-like. Plotting these values over time and taking the upper envelope yields a nice march towards habitability.
Using a simple bootstrap sampling analysis, we calculated when a logistic curve fit to such an upper envelope would get to a habitability of approximately 1. And the likeliest time is early to mid-2011, or more precisely, early May 2011.
And if not then, their data shows that there is a 75% chance that the discovery will happen by the end of 2013 (many astronomers predict this timeframe as well). In February 2011, NASA's Kepler mission will release a boatload of data that I'm assuming will help in find this Earthy planet. The universe is so big I'm surprised I didn't realize discovering another Earth-like planet was inevitable. There goes thinking that I'm a unique snowflake.