How close are we (and have we been in the past) to an actual outbreak of this disease in the U.S.?

There still has never been a case of Ebola in the United States (in humans). Thus far, they've been able to keep the virus contained in Africa, although there were a number of monkeys brought into the states from the Phillipines that were in fact infected. Some workers became infected through these primates, but they did not become sick.