It seems to have started with this:
According to news reports, the ban applies to flights to and from 8 countries, but there's no official announcement - so no list of countries affected.
Saudi airlines have confirmed the ban applies to them. Presumably this is in response to some known specific threat. I guess a concern is TATP can't be picked up by airport scanners, and the bad guys seem to be getting better at making and using it. But if concerns about quality of screening/security at those foreign airports is the issue, that doesn't really explain why outbound flights from the US are also affected.
Could be rather upsetting if you had to surrender your new camera or ipad at check-in - possibly never to be seen again.
I hope it doesn't spread - increase to cover more destinations / departure points.