There were some very good comments on, "Is Exponential Growth Possible?." Kimsia linked to this interesting academic paper on learning - http://acs.ist.psu.edu/papers/ritterS01.pdf
It's fairly intuitive and not groundbreaking, but there's a few really interesting points in there.
The basic idea behind the paper is that the more you practice a skill, the faster you get at it - but the gains slow down and flatten out over time.
The pattern is a rapid improvement followed by ever lesser improvements with further practice. Such negatively accelerated learning curves are typically described well by power functions, thus, learning is often said to follow the "power law of practice". Not shown on the graph, but occurring concurrently, is a decrease in variance in performance as the behavior reaches an apparent plateau on a linear plot. This plateau masks continuous small improvements with extensive practice that may only be visible on a log-log plot where months or years of practice can be seen. The longest measurements suggests that for some tasks improvement continues for over 100,000 trials.