In the past, I've covered this topic in quite a bit of detail, e.g. see this subset of links:

I think that, collectively, the material in these links make a very strong case against the speculation hypothesis, but I should be careful to clarify my view. This is from the fifth link on the list, Oil Prices and Speculation (for a more technical treatment, see here and here, i.e. the first two links on the list):

We’ve heard a lot about how speculation has caused volatility in oil and other commodity prices recently, and there are calls in Congress to put constraints on speculative activity in order to stabilize prices and markets, so let's go back to the issue of whether speculative activity has been the driving force behind commodity price movements, oil prices n particular.

To begin, it's important to recognize that not all speculative activity is the same, and not all of it is bad, and as we look into how to better regulate these markets, we need to keep the types of speculative activities separate so that we don’t stifle the good type of speculation as we try to eliminate the types that cause us troubles.

First, speculative activity can arise from attempts to profit from manipulating the price of a good, and some people believe this type of manipulative activity can explain much of the volatility in oil prices we have seen recently. This, obviously, is a bad type of speculation and we should prevent it to the extent possible.

Second, moral hazard combined with easy money can lead to an undesirable type of speculation. If market participants have ready access to funds, and if they believe losses will be covered, say, through a government bailout, then they may be willing to invest far more than is optimal in speculative ventures. If they hit it big, they win. If things go sour, the government covers their losses.

A third type of speculation we’d like to avoid is speculative bubbles, and this is probably what most people have in mind when they hear the term speculation. Speculative bubbles occur due to “bandwagon effects” where rumors or some other force causes prices to deviate from their underlying fundamental values in a self-feeding frenzy that drives prices upward in a bubble, or downward in a crash.

Fourth, speculation allows us to insure against expected future changes in supply or demand, that is, anticipated changes in the price. If we expect higher demand or lower supply of a good at some point in the future, that is, if we expect a higher future price, then speculators will take some of the good off the market today, store it for the future, and then sell it after the price rises. In this way, speculation provides insurance against the future fall in supply or increase in demand by having the good available to meet those changes.

Finally, there is stabilizing speculation, for example selling short near peaks in anticipation of price declines can dampen natural market volatility, and this is generally desirable. This type of speculation - short-selling - is under considerable scrutiny right now, but in general this dampens rather than enhancing market volatility and we ought to encourage the dampening variety.

So yes, by all means, limit the bad type of speculation through regulatory changes. But be sure to keep the types that help.

I've taken the stance that there is little evidence of the first and third types of speculative activity, manipulation and bubbles divorced from fundamentals, and I don't think the second type - moral hazard - made a large contribution. I've argued fundamentals are the most likely source of most of the price variation, and by fundamentals I mean any new information that causes people to change their expectations of supply and/or demand, and I've taken a lot of criticism here over that stance.