MacBook Pro 15" Retina Display Mid 2012 Teardown
As for the soldered-in RAM: I believe that they've figured out what the probability of failure of RAM is, and it must be low enough that it's not likely that RAM will be failing more often than anything else on the board. If you want replaceable RAM, why stop there: you could have a replaceable CPU. And GPU! And why not something else as well. See where it goes? RAM is not special in any way.