I've yet to read through the actual cables, but The Guardian's summary doesn't paint the prettiest picture:
The US fears that Saudi Arabia, the world's largest crude oil exporter, may not have enough reserves to prevent oil prices escalating, confidential cables from its embassy in Riyadh show.
The cables, released by WikiLeaks, urge Washington to take seriously a warning from a senior Saudi government oil executive that the kingdom's crude oil reserves may have been overstated by as much as 300bn barrels – nearly 40%.
According to the cables, which date between 2007-09, Husseini [a geologist and former head of exploration at the Saudi oil monopoly Aramco] said Saudi Arabia might reach an output of 12m barrels a day in 10 years but before then – possibly as early as 2012 – global oil production would have hit its highest point. This crunch point is known as "peak oil".
Husseini said that at that point Aramco would not be able to stop the rise of global oil prices because the Saudi energy industry had overstated its recoverable reserves to spur foreign investment. He argued that Aramco had badly underestimated the time needed to bring new oil on tap.
UPDATE: A more cornucopian take on this whole saga from Michael C. Lynch here. Although, it would be remiss of me not to show you this Forbes article from 2006, where Lynch was confidently predicting that oil prices would dip to $20/bbl by 2008...