Looks like the happy occasion occurred last night!
Here's the AP's take on the tale.

Knox Leon and Vivienne Marcheline are healthy and well according to reports -- as are the new twin mum and dad. :) [Love that they paid homage to Angelina's recently deceased mother with their new daughter's middle name.]

And so much for all those twin girls rumors, eh?

Welcome to the Boy-Girl twin parenting crew, Angelina and Brad!
All the very best for a life of continuing multiple blessings....

I cannot wait to see the pictures! My money's on People magazine scoring the scoop... Can you imagine how gorgeous this family is going to be? Wow...

Congratulations to the Jolie-Pitt Family!

And of course, it's only fair to vote on these baby names, too!

What fun, what fun! I love their huge crazy celebrity family! Somehow, I really do believe that Brangelina are sincerely just doing their own thing, trying to save the world, and enjoying the blessings of parenthood. 

Love the name Vivienne... but Knox, not so much. I get that they're trying to keep all their sons' names X-rated, but Knox is just a bit too Fox in Socks for me, I guess :) 

 OMG! i know! i cant wait to see their kids either theyre going to be SOO beautiful just like their parents...Im with people as well because theyre usually the ones that get these things first lol...I cant WAIT!

I'm quite sure the kids are pretty.  I like the daughters name.  I think Knox is different.  Its something I would have to hear more often.  It's better than the actor or musician who named his son Pilot Inspektor.