They are here. They were first discovered in Texas in 1990.
Here's a map of sitings in California: http://bees.ucr.edu/ahb_CA_05.pdf
More details: http://bees.ucr.edu/ahb-facts.html
All the hype aside, beekeepers have found ways to mitigate the effects. The primary problem with the Africanized bees is that they are more easily provoked, and more hostile. An individual sting isn't any more dangerous than a "regular" bee, but they are more likely to swarm someone they perceive as a threat.
More info here: http://nationalatlas.gov/articles/biology/a_bees.html
btt