MacBook Pro 15" Retina Display Mid 2012 Teardown
All this is no different than a MacBook Air. The SSD is replaceable in the previous MBAirs with faster, higher capacity ones from a number of vendors. It's a good bet that this one (and the current MBAirs) will be the same.
Think of it as a MacBook Pro, weight optimized like a MacBook Air. Or a fusion of the two. You can always get the regular MacBook Pro with optical and upgradeable bits if you don't like this way of building things.