For some reason that we haven't been able to figure out at Opera Forums, Opera 12 x64, starting with the RC1 build and continuing through the public release, won't plant itself in my Default Programs > Set your default programs list. (edited)
Update: Solving this required uninstall of Opera and completely cleaning registry of all Opera mentions (except related to other files), then reinstall. Not sure it the cause was a bug or something with my (and some others') system.