There were places out west that were doing that at gas stations. Of course if it dips then you buy your gas elsewhere until it goes higher than what you paid for it. Same thing can be done with home heating oil in fact a friend of mine put in another oil tank in his basement before the end of his contract last June and had that tank also filled up at the low price from 2007 summer that he had. Of course now the price is even lower but who could know that.
I used to do something similar... I purchased gasoline price protection last year. My I was locked in for 19 gallons/week @ 3.07/gal. I was really reaping the rewards when my protection ran out the last week of June. The downside is that I still haven't been paid for the last month, but I was in contact with the company last week, and they said they will send me a check for the last month's price protection in the mail.
A few years ago Pricewatch.com was doing something similar with gas, where you bought the gas up front, and used it later. They don't do that anymore either.