Does the new Macbook Pro with Retina display have user-upgradeable RAM?
I called Apple to find out, and they confirmed that it is not user-upgradeable: the tech explained that they used the same process they used for the MacBook Air to save space: there are no sticks of RAM, and the RAM has been soldered directly onto the main board.

As Russell Borogove noted in the comments, this is now confirmed when you configure the MacBook Pro before purchase:


  Please note that the memory is built into the computer, so if you think you may need more memory in the future, it is important to upgrade at the time of purchase.


So if you were thinking you might want the 16GB RAM in the future, you'd want to choose it when you order the MacBook Pro, as you won't be able to upgrade later.