Do I need to take into account future RAM usage when customizing my MacBook Pro order, or can I go with 8GB RAM and upgrade it later?
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.
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.
iFixit has posted a tear-down of the new Macbook Pro with Retina display and on step 17 you can see that the RAM is soldered to the back of the logic board (red rectangle):
Picture from iFixit.com
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