It's hard to comment on the specific likelihood, but a few general observations:
1. Ebola transmits via bodily fluids (blood, mucus). Plus, Ebola does not establish a chronic infection in a host. Pathogens with those characteristics usually don't spread that fast / wide.
2. Past observations support 1) - transmission in airplanes has never been observed AFAIK.
3. Infected people can transmit the virus while having a fever, and through later stages of disease. This means you can identify potentially contagious people quickly, which helps control efforts.
4. Having said that, there are lots of things we don't know about Ebola, and it's always best to plan for the worst.
The diseases that keep me up at night are those that spread via airborne or vector transmission & have lots of asymptomatic carriers. Ebola has neither of those. Nevertheless, as far as Ebola outbreaks go, this one is the worst yet and correspondingly worrying.
The primary positive is that the disease has a very quick run from infection to visibly symptomatic to death. While long distance spread is still possible, most outbreaks have been somewhat geographically contained because of this. The worst case scenario for deadly diseases is asymptomatic carriers, and I don't know if there is such a thing with ebola (except for the first few days of infection). It's why diseases like HIV have ultimately had greater impact. It can spread for years before anyone becomes symptomatic.
The virus can take up to 21 days to become symptomatic. More worrisome is that it's been found in fluids up to 60 days post infection. If the flu can spread widely, so can this -- the main limits thus far have been that it doesn't spread via the air, and that it's been geographically isolated. You can't call this outbreak geographically isolated anymore.
Unfortunately, this oft-repeated description is wrong. Ebola has a weeks-long incubation period and is contagious throughout most of the incubation period. Plenty of time to spread the virus before the host becomes symptomatic.
