Actually,
reticulating splines is the process of combining a series of short lines and curves into a longer
spline or network of splines. It's a process used in the landscape generator in many computer games. Basically, the engine generates a few small patches of terrain and then
interpolates between these.
However, because of the female voice enunciating the phrase at the beginning of SimCity 2000, the phrase has entered the lore of the Sim games. Since then, it's been seen in SimCopter and The Sims, and possibly several others.