I read recently that scientists have estimated there is only about 40 years left of fuel on our planet. Is this true?

These kinds of estimates have been getting bandied about for decades. It started with The Population Bomb by Paul R. Ehrlich in 1968. In this book, Ehrlich predicted that we would run out of numerous resources with in a very short period of time leading to famines and other disasters. Every one of his doomsday predictions has proven wrong, but this did not stop others from listening to him and making similar predictions of their own.

The fact of the matter is that these estimates of how much energy we have left in the world are based on _known_ reserves for fuel. That is, they are based on the deposits of fuel that we have actually found and know are there. If you look at the estimates of how much oil we have available to us over the pass several decades, you will see that they consistently hover at around enough to last us for about another 30 years. However, despite the fact that they have been saying this for the past several decades, have you noticed that we haven't actually run out? The reason is because we keep finding more of the darn stuff. Earlier this year, Mexico found a major new reservoir of oil. The fact of the matter is that there are still lots of places that could potentially have oil that have not been fully explored yet. So, there is still the potential for a lot more oil to be found.

Now I focussed on oil above because that 40 year estimate that you cite can't possible take into account other fossil fuels. The U.S. has enough coal to provide for its energy needs for the next 200 years. There is a process that was developed by Nazi Germany by which we can even turn this into automobile fuels. We have not done so because the process is rather expensive. However, if oil does run out and/or oil prices go too high, then we can switch to this.

So, don't let those that are telling you that we are running out of energy scare you. This is not the case.

This number is highly debated, but most such estimates are based on projected energy demands, without accounting for sudden icreases in demand like China's sudden thirst for oil, or the impacts of unforeseen technological advances.

It is almost universally agreed that, if those projections for future demand are accurate, we will likely see our world oil supplies dry up before the end of the century unless we take measures to conserve our dwindling natural resources.

However, there are some people that believe that the oil supply is infinite and 'prove' it to you by driving the largest, least fuel-efficient vehicle they can buy. I like to call these people "Americans" :)

I dont think that is true at all, you need to go see how much untapped oil is under Iraq and thats just Iraq.
Then people wonder why we went to Iraq, the answer is obvious.

That sort of prediction was made at the same time they were talking about peak oil. Its affect was twofold, first it raised the price of oil, second it gave an incentive to people developing alternative sources.

Since then, new fields have been discovered, one of the largest future natural gas supplies was discovered right here in Pennsylvania, and other energy sources are becoming popular.

Oil will not disappear as a major power source until the energy suppliers figure out a way to charge for solar and wind energy. Remember all of this is driving by money, not conscience, or desire to save the world. Money and power always prevail.

This sort of comment usually has a qualifier or two when originally used. Something like 'of cureently confirmed oil fields'. These always get missed out and used in a very simplistic manner. Just the known shale-oil eserves in the arctic would last over a century, but they ae only just economic to extract. Plus, no one has looked at the antartic yet !
So the simple answer is; NO, it's not true.

That is ten years more than what they were calling for thirty years ago. So if they are right, that is a total of seventy years more than what they were calling for in 1972.

With the advent of breeder reactors nuclear power will give us 200 years of energy based on an exponential curve (taking into account average reactor efficiancy change per year). However within that time It is likly that a more efficient "free" power sorce will be created taking over for the currently used powers. Allthough such a energy sorce is currently based on speculation it is likly that given 200 years it may very well be posible to create one hence solving the problem of power demand.

If it is true I sure hope we get to where we are going soon :)

"In 2005, the United States Department of Energy published a report titled Peaking of World Oil Production: Impacts, Mitigation, & Risk Management. Known as the Hirsch report, it stated, "The peaking of world oil production presents the U.S. and the world with an unprecedented risk management problem. As peaking is approached, liquid fuel prices and price volatility will increase dramatically, and, without timely mitigation, the economic, social, and political costs will be unprecedented. Viable mitigation options exist on both the supply and demand sides, but to have substantial impact, they must be initiated more than a decade in advance of peaking."

- Conclusions from the Hirsch report and three scenarios
After careful analysis, the Hirsch report came to a number of conclusions:
World oil peaking is going to happen - some forecasters predict within a decade, others later.
Oil peaking could cost economies dearly - particularly that of the U.S.
Oil peaking presents a unique challenge - previous transitions were gradual and evolutionary; oil peaking will be abrupt and revolutionary.
The real problem is liquid fuels for transportation - motor vehicles, aircraft, trains, and ships have no ready alternative.
Mitigation efforts will require substantial time - an intense effort over decades.
Both supply and demand will require attention - higher efficiency can reduce demand, but large amounts of substitute fuels must be produced.
It is a matter of risk management - early mitigation will be less damaging than delayed mitigation.
Government intervention will be required - otherwise the economic and social implications would be chaotic.
Economic upheaval is not inevitable - without mitigation, peaking will cause major upheaval, but given enough lead-time, the problems are soluble.
More information is needed - effective action requires better understanding of a number of issues.
The report listed three possible scenarios: waiting until world oil production peaks before taking crash program action leaves the world with a significant liquid fuel deficit for more than two decades; initiating a mitigation crash program ten years before world oil peaking helps considerably but still leaves a liquid fuels shortfall roughly a decade after the time that oil would have peaked; or initiating a mitigation crash program twenty years before peaking appears to offer the possibility of avoiding a world liquid fuels shortfall for the forecast period."
Source and further information:
http://en.wikipedia.org/wiki/Peak_oil

Further information:
http://en.wikipedia.org/wiki/World_energy_resources_and_consumption

They were saying that 40 years ago.

Maybe as far as American fuel reserves. Canada, for one, has more than enough for it's own needs as well as exports for at least another 100 years.