Flipping a lesson seems to mean different things to different people these days. But here is one definition, via Knewton, that makes a lot of sense. Instead of having the teacher convey information/explanations to the learners during classroom time and then getting the learners to practice it later for homework, the learners watch/listen/read the lecture or explanation or teaching part of a lesson at home and then come to the classroom to try it out, practice it, or get feedback on performance. That makes a lot of sense to me. Many teachers are already doing some form of blended learning these days, usually either parking resources on the web or providing learners with options for web-based self-access learning. Flipping a classroom seems to be the next logical step.
Of course, there are issues that come to mind immediately–Internet access inequality, digital learning literacy problems, motivational differences, and (for many teachers) just adjusting to a new way of approaching teaching. But the idea is interesting, the technology is extant, and I believe most learners are very, very ready for this. And the new official course of study in Japan comes into effect for high school this upcoming April, requiring more English in the classroom, particularly more productive use of English by learners. This may be one way of really achieving the directives.
March 25th update: Recently, Blackboard posted the slides of presentation on the challenges of flipping classrooms.