The possibilities for difficulties in US-Mexican relations are enormous. If the "conservative" wins, he is likely to adopt policies that will lead to increased emigration to the USA, as well as brutalizing his own populace. Mexican law enforcement is already harsh and corrupt; it would likely get worse under Calderón. If AMLO wins, on the other hand, US action against Mexico seems likely, though exactly what form that action would take is not clear to me; perhaps covert ops as was undertaken against Allende in Chile. There don't seem many positive possiblities, though the Mexicans could surprise us. I hope they do; if not, things seem grim indeed.
Best coverage on the net I am aware of is at The Agonist; the San Antonio Express-News and KENS, San Antonio television channel 5, has an ongoing blog.