I often joke with the people I'm training in export compliance that it's easy to tell when a country's about to come off the US embargo list -- once the bombs begin to fall it's usually a good bet that American sanctions will be the next thing to be dropped (see Iraq, Afghanistan, Serbia, etc.).
Predicting what could trigger a new US embargo isn't generally as obvious, but I challenge you to read this story on Venezuela's efforts to counter American hegemony in Latin American and beyond without at least considering what it might take for Chavez to garner an embargo of his very own. (Fidel would be so proud.)
LATE UPDATE: That didn't take long -- an arms embargo on Venezuela (a measure still well short of comprehensive sanctions) will take effect on October 1. For more on el presidente, I recommend Franklin Foer's profile in the May Atlantic (subscription required).