Non-retina 13" and 15" MBP, 2012:
RAM is user upgradeable.
HDD is user upgradeable.
Battery is not considered user replaceable by Apple but you can remove it with the right screwdrivers.
Retina 15" MBP:
RAM is not upgradeable, physically soldered in.
SSD/Flash is not considered user replaceable by Apple, but it's a small board that can easily be replaced if someone starts offering replacements.
You need a special screwdriver (pentalobe) to take off the bottom. It isn't glued shut.
The battery is not replaceable (glued into case).