No offense Darkeryet, but it is clear that you know little about Mexico. It is a soveriegn nation, with its own history, culture and pride. Most Mexicans are happy being Mexican and have nationalistic feelings just like most U.S. citizens. Yes, their economy is not the strongest and they are struggling to clean up their government and law enforcement, but I would imagine that a vast majority would rather not become part of the United States. I have spent a fair amount of time in Mexico and know a number of Mexican families and while they generally like Americans they dislike to patronizing attitude America has towards Mexico.
On the flip side, I really do not think that the U.S. would want to take Mexico at the 51st state. It is a large, poor, Spanish-speaking nation that would take trillions of dollars to bring up to U.S. standards.
Area: 1,972,550 sq km - slightly less than three times the size of Texas!
Population: 100,294,036 (July 1999 est.) compared to the U.S. population of 278,058,881 (July 2001 est.). That would increase the U.S. population by approximately 36%!
Absorbing Mexico would put an enormous strain on the U.S. economy and would reshape American/world politcs.
Edit: I got my facts from Geographic.org:
http://www.geographic.org/countries/