The range of dates varies from 2005 i.e. we're already past the peak, to 2032 (US geological survey). The answers vary for a variety of reasons. There are disagreements on: how much oil there is in the ground, how the oil forms, how much can be extracted, what reserves various countries are sitting on, how depleted those reserves are. Some people base their figures on light sweet crude only, while others factor in tar sands, deep ocean oil, liquids from coal etc.
The inescapable fact though, is that we use about 28 billion barrels per year of the stuff (about 1 cubic mile). The age of cheap oil is rapidly ending and it's only going to be harder from now on unless we find a few more Saudi Arabias. Try www.peakoil.com or www.theoildrum. Some of the discussions are quite hair raising though. The sites attract a generally literate, scientific bunch with the odd survivalist or religious loony thrown in. Some of those making the most dire predictions are some of the geologists and engineers with the most detailed knowledge of the global oil industry.