UI will open up(If not all open but most of it anyway):
Zypher project, it will eventually though...
And as to those pondering over the UX, it hasn't been decided to what extent the entire UX will be open/closed, it depends on how their licensing is fully fleshed out.
It may turn out that most of it's open, I think what will happen, is that Jolla will keep some bits closed, but keep most of it open for all Alliance members to use/extend.
And Sailfish is not really a fork of MeeGo, it is *kinda* a fork of MeR+Nemo....
MeeGoCE (not Harmattan) was barely advanced before all resource were dropped at the start of 2011, they've just plodded along with the same code-base.
I guess you could call MeR+Nemo (on which Sailfish is based) a fork of MeeGoCE, but it's less so than MeR+Nemo -> Sailfish.