I still don't know what the hell a commonwealth is :sadvaultboy:
Sorry for digging this dead post back up...
The commonwealth is the location where all the smartest minds have gathered to do research and what not. At least, thats what i was told by a scientist in Fallout 3.
I would love to be able to see the Commonwealth. If even half of what that guy told me was true, it would be a site to behold indeed.