Rising oil prices — the great villain of 2008 — are back, but this time markets are cheering.

Oil futures jumped over $71 a barrel this morning, boosting shares of Exxon Mobil and Chevron — and the Dow overall — as investors looked towards the rally in recent months as a bullish sign that demand is returning to an ailing global economy.

Or is it?

Separately today, the Paris-based International Energy Agency slashed its forecast for world oil demand over the next five years, saying that by 2013 global demand will average 87.9 million barrels a day, 3.7% fewer than it expected in December and 7% fewer than it expected last July. The group predicts oil consumption will fall by 3% this year, the sharpest decline in a quarter-century, after averaging about 2% growth annually over the previous decade.

Why all the fuss over oil demand? Because it’s critical to know what’s behind rising oil prices. If it’s demand — if nations like the U.S., China and India are consuming more oil because they’re building infrastructure and powering factories — then indeed higher prices are a bullish sign for global economic growth, even if it means some grumbles at the pump.

But if it’s not demand at play, as the IEA’s outlook seems to suggest, then rising oil prices start to look a lot more sinister — especially because higher oil prices take a chunk out of consumers’ pocketbooks and companies’ earnings, a scenario that unfolded to much dismay last summer, when prices topped out in July at over $145 a barrel.

One possibility is that supply could be too tight; that producers have slashed their output too much. But if that’s the case, it would seem they’d now be eager to pump more oil through their pipelines and cash in on the higher prices (which then helps to stabilize prices at that elusive “equilibrium”).

Or, it could be speculation. It does seem a little odd that global supply and demand would have gyrated enough over the past year to send prices soaring last summer; then crashing; and now rebounding swiftly even as the world economy navigates its worst downturn since the Great Depression. Last summer, Richard S. Eckaus, a professor of economics at the Massachusetts Institute of Technology, published a short, easy-to-read paper with the Center for Energy and Environmental Policy Research titled “The Oil Price Really Is A Speculative Bubble”.

He writes that “there seems to be a preference for the claim that the price increases are the result of basic economic forces: rapid growth in consumption, pushed particularly by the oil appetites of China and India, the depreciation of the U.S. dollar, real supply limitations, current and prospective and the risks of supply disruption, especially in the Middle East.” He briefly explores — and debunks — each of those possibilities, writes that the price of oil is behaving much like any other speculative bubble, and encourages efforts to “break the bubble” given its adverse effect on the economy.

The latter suggestion makes for an interesting debate: if oil price fluctuations are being driven by trading activity rather than supply and demand, should such activity be regulated, prohibited even? Only one thing is certain: the faster and higher prices go, the more plausible the idea will become.