According to Wall Street Journal article of February 9, 2006 (subscription only) "Mexico's huge state-owned oil company [Pemex] may be facing a steep decline in output that would further tighten global oil supply and add to global woes over high oil prices". An internal report, which served as basis for the Journal's story, covered several possible scenarios of declining production from one of Mexico's larger oil "pools". The worst scenario looked at would lead to a reduction of about 63% of Mexico's daily crude exports to the U.S within two years, according to the Journal. Because Mexico is the US' second most important supplier, even a 'moderate case' scenario could be serious for both countries.
Peak Oil happens one field at a time. Sure there are many small fields left in Mexico and many more in the rest of the world; but, the total projected return on investment to extract from each of the smaller fields individually is less. That means that as the "big" fields head into decline, that the cost of goods increases much faster than it has in the past
For a look at past TreeHugger posts on Peak Oil, the following list of links is a handy starting point.
Peak Oil Report: Half of Kuwait Oil Reserves Disappear
Norwegian Peak Oil?
Kunstler: Oil Prices To Rise Sooner Than Later
Grist Interviews Matthew Simmons on Peak Oil
October 5th: Petrocollapse Conference In New York
Peak Oil — The Lessons of Y2K
Chevron-Texaco Pulls a "Beyond Petroleum"
See Peak Oil for Yourself
Ascent of Peak Oil