The Kicking Tires blog on cars.com points out:
Turns out, 33 cents in one week is the biggest price drop in the history of anyone paying attention to the national average price of a gallon of gas. Currently, the national average price is $3.12, according to AAA. With the price of oil below $80 a barrel, this price could come down even more over the next few weeks, bringing $3 gas back into sight.
And EconomPic Data has the pretty illustration:

October 21st, 2008 - 9:00 am
I guess the oil companies were just so scared of Obama’s windfall profits tax threat that they decided to cut their prices. Now that’s change I can believe in.
October 21st, 2008 - 1:52 pm
Silly me, I thought it was something ridiculous like decreased demand.