In researching another thread, came across this interesting article on preferments from Lallemand, in PDF format [1].

One interesting morsel:

The preferment minimizes the lag phase by providing an optimum environment for the yeast. The result is higher gas production later inthe process, especially in high-sugar doughs.

The lag phase is the "ramp up" phase that occurs before yeast reach their maximum productivity. The article has a nice chart. 

Here's another interesting one:

Yeast activation takes place during the first 30 to 60 minutes in all types of preferments. Longer preferment times are not necessary for yeast activation, and can have a negative effect because yeast start to lose activity once the available sugar has been consumed. The only reason for longer preferments is for flavor contribution or dough development.

I think they're referring to the activation of commercial yeasts here (Lallemand is a commercial yeast producer, after all). Yeast activation is sourdough I think is different altogether.