Yep, I think it's something like 3% can take up to 42 days to show "symptoms" and believe it or not, some people never do show symptoms.
Apparently that's how Duncan got Ebola. He took his neighbor (whom no one thought had Ebola because there were no symptoms) to a "non-ebola" hospital but they were full so he took the neighbor home. Not sure if the neighbor died before or after he left the country, but either way, they didn't know, until the tests came back positive, that the neighbor did indeed have Ebola. But at that point Duncan was already in the U.S.
There's also, from what I've read, issues with "false negative" tests. The W.H.O. (because of this issue) recommends 2 tests done 48 hours apart. But, as I'm sure we're all aware, this isn't being done here.
Just read earlier, a story about a guy who traveled from West Africa taking himself to the hospital to be "tested" because he was afraid of having Ebola ... Not because he was showing symptoms. What good does testing do, if you're not showing symptoms yet? How accurate is that test then? Apparently they tested him, said Nope, No Ebola and sent him on his merry way.