It's been covered before, but Fedora is the recommended/standard distro. This is because we NEED a standardised distro for the educational release - we need people to be able to say -"We are using your distro and your stuff doesn't work", rather than "I've got this random distro I made and your stuff doesn't work". In the first case we know we have a problem. In the second case, all bets are off - could be anything.
That said, we heartedly encourage people to use whatever distro they want. Just don't expect top level tech support from the Foundation itself, on Foundation released packages. We'll try, but we cannot promise anything on other distros, the support burden is too high.
I appreciate all the work the Foundation has done, but I don't think it is very realistic to think they are going to be able to furnish tech support for the hundreds of thousands of Pi's being sold.
And I'll readily grant that the UK educational system, which should, and probably will be, the support focus of the Foundation, should have an "officially supported" distro. Or better yet, a very limited "set" of distros. But it should still be kept in mind that encouraging the students to "widen their experience", by NOT limiting THEMSELVES to only one distro, should be the ultimate goal.