It's Alive! and Jelly Bean Flavored!
Merge will commence when source is fully downloaded; this means hours not minutes since people all over the world will be hitting these servers.
Do not expect nightlies to immediately be available for JB, we will have them up once we are content with where the ROM is.
As noted before, this does not change the plan for CM 9 - we anticipate an RC2 and then the stable release.
Big shout out to +Jean-Baptiste Queru
and the rest of the Android team.
Let the fun begin!