Sadad Al Husseini disagrees with Simmons' assessment:
Although Matthew Simmons says it is unlikely that the Saudis will be able to produce 12.5 million barrels a day or sustain output at that level for a significant period of time, Husseini says the target is realistic; he says that Simmons is wrong to state that Saudi Arabia has reached its peak. But 12.5 million is just an interim marker, as far as consuming nations are concerned, on the way to 15 million barrels a day and beyond -- and that is the point at which Husseini says problems will arise.

Who is Sadad Al Husseini?

It can be argued that in a nation devoted to oil, Husseini knows more about it than anyone else. Born in Syria, Husseini was raised in Saudi Arabia, where his father was a government official whose family took on Saudi citizenship. Husseini earned a Ph.D. in geological sciences from Brown University in 1973 and went to work in Aramco's exploration department, eventually rising to the highest position. Until his retirement last year -- said to have been caused by a top-level dispute, the nature of which is the source of many rumors -- Husseini was a member of the company's board and its management committee. He is one of the most respected and accomplished oilmen in the world.

Source(pdf)

Perhaps Sadad knows, but is he telling the whole truth?  Would he risk his head for a NY Times reporter?
Personally, I doubt it.

I get the impression from reading "Twilight in the Desert" by Simmons that Saudi Arabia will be lucky to maintain the 9.5 - 11 mbpd level of output they currently have for much very longer. They will need to discover some significant new fields in order to raise production by 4 to 5 mpbd (to get to 15 mpdb), esp. once depletion becomes evident in Ghawar and their other large-but-aging fields.

Also today, reference to Simmons in the NY Times, via the International Herald Tribune (no reg required):  

Is the world running out of oil? Yes and no

'We're halfway through the hydrocarbon era," my old friend Boone Pickens has been saying for the last couple of years. A folksy line like that, it sticks with you. But I hadn't realized until I began poking around the world of oil forecasting in the wake of Hurricane Katrina that it also meant Pickens had taken sides in a surprisingly heated debate. He subscribes to what is sometimes called the "peak oil hypothesis," which holds that there simply isn't very much new oil left to be found in the world. And, as a result, we are currently in the gradual process of draining the more than 1.2 trillion barrels of proven reserves that are still in the ground. And when it's gone, it's gone.

http://www.iht.com/articles/2005/09/09/yourmoney/mjoe10.php

What this does not say, and elides over, is the fact that as you slip down the other side of the peak oil curve, the energy cost of extraction increases dramatically, and you would be lucky to be able to extract half the 1.2 trillion barrels left