The "spaced repetition" theory is similar to something I read in Wired[0] long ago, of a guy and his note-taking "SuperMemo"[1] app. The article drifts aimlessly about the guy, but the theory behind the app is very interesting. And the chart is excellent[2].
For example, say you're studying Spanish. Your chance of
recalling a given word when you need it declines over
time according to a predictable pattern. SuperMemo tracks
this so-called forgetting curve and reminds you to
rehearse your knowledge when your chance of recalling it
has dropped to, say, 90 percent. When you first learn a
new vocabulary word, your chance of recalling it will
drop quickly. But after SuperMemo reminds you of the
word, the rate of forgetting levels out. The program
tracks this new decline and waits longer to quiz you the
next time.
[1]: http://en.wikipedia.org/wiki/SuperMemo
[2]: http://www.wired.com/images/article/magazine/1605/ff_wozniak...