Over at iFixIt.com is a very nicely done iFixIt teardown of the MacBook Pro Retina.
Looks to me like there is one (1) thing that will be upgradeable: the SSD, which is on an easily removable daughtercard. So I expect to see 3rd-party alternatives pop up fairly soon, perhaps even with capacities up to 1TB (maybe, might be limited to 768GB).
The memory is soldered on as I previously discussed— no way to upgrade the memory, so avoid the 8GB model like the plague.
Also worth noting is that one of the many internal battery modules could have been left out in favor of a 2nd internal drive module. Admittedly, that would not pass the 90% rule: 90% (or maybe 99%) of users would never add a 2nd drive. The tradeoff is a function of design priorities, not inherent technical limitations. So it’s hard to argue with that choice.