To buy futures means that you're temporarily taking a product off the market and selling it later. As many people have pointed out the process increases the price of said commodities by taking the product off the market and decreasing the supply. Some economists have speculated that much of the recent increase in oil prices has been due to precisely this very process.
The process gets scary around here. As Ben Stein illustrated the first impulse of many investors once the stock market declines is to shift that money to futures, specifically oil futures, because right now that's where the money is. But investing in oil futures increases the price of oil which drains the American economy, which hurts the stock market, which could potentially lead to more people shifting to oil futures, which would drain the American economy, which hurts the stock market -- and on and on and on. Throw in a declining dollar which takes a blow every time the stock market declines, thereby pushing up gas prices even further, and you have a cycle in which the American economy is literally eating itself from the inside. It's a constant flow of money out of the stock market, out of business, into higher gas prices.
There are of course a couple of qualifying remarks to make. It's hard to say with any degree of certainty what percentage of investors, dismayed with the stock market, shift to oil futures, or whether it will actually manifest itself in a continually compounding phenomena like what I described. There's also the widely held belief that the oil futures market is a huge bubble likely to collapse at any moment. It almost certainly is the former. The latter, we can hope, is true though there are bound to be negative economic ramifications to the bursting of that bubble as well.