I have some numbers to share that may help. I've been tracking my home's natural gas consumption in a spreadsheet for years. Much of that time I'd only been interested in the quantity used – to measure my home's efficiency after certain upgrades – but in 2006 I also started tracking the "Gas Supply Charge" costs from my local utility, Enbridge, in Ontario, Canada. My numbers are for the gas commodity only (i.e. excluding delivery and customer charges.)
I've never been on a fixed-price contract, so the numbers are supposed to be reflective of market rates. However, the numbers do differ from real "spot prices" because Enbridge estimates gas costs up-front and then applies a "gas cost adjustment" at later dates if their estimate was wrong.
Natural gas cost per cubic meter for Chris's home http://img686.imageshack.us/img686/6406/naturalgascosts3priorye.png
Since 2006, natural gas prices have been generally falling. The last cost I have on file, from my November 2009 bill, is 12.9 cents per cubic meter – being ~20 cents gas supply rate, less gas cost adjustment of ~7 cents. My average cost over that nearly 4 year period, January 2006 through November 2009, was 38.4 cents per cubic meter.
Considering the current 5-year fixed rate I found is about 29 cents per cubic meter, there is a substantial premium to locking in when compared to current market rates. However, one can see that during the last 4 years, market prices did substantially exceed that rate for quite some time. Furthermore, when I last looked at those 5-year fixed rates perhaps a year or more ago, I couldn't find a company charging less than 39 cents per cubic meter. So, contract rates have fallen as well.
Consequently, if we are at a natural gas price low and the economy is to recover, I tend to agree with Cart's answer and suggest it could be a good time to consider a fixed-rate contract. But, do your own due diligence and read the fine print if you go for it.
UPDATE: In the interest of full disclosure, shortly after I did my own research above, I signed up for my first ever fixed-rate natural gas contract. :-)