Let me try to summarize: 1. Articles are written by experts, or are copied from "free encyclopedia sources" 2. They get checked and maybe expanded by experts 3. They are stored in a "safe" place where every change, if any, is controlled 4. These modified articles are a "free encyclopedia source" again 5. As a result, these articles (or parts of them) can be integrated into the "free encyclopedia sources" from #1
So, Wikipedia could be the basis for Nupedia (which is no problem in itself; I could open up "Magnuspedia" today, based on Wikipedia, and declare it expert-edits only, and noone could stop me). But, whatever the experts at Nupedia will come up with, it will probably be better than the corresponding Wikipedia article it is based on. Wikipedia can only profit from such edits, as they can be used in turn. There's one thing Wikipedia will always beat Nupedia in: Growth. At the moment, that is growth in the number of articles. Bu, at some point, it will be growth of individual articles. Many articles I originally submitted to Nupedia have been growing enormously on the fertile soil of Wikipedia. Articles can grow on Wikipedia, be proof-read in Nupedia, and gan then grow further on Wikipedia again. What I'm trying to say is Wikipedia and Nupedia won't be a fork, because they're not really competition; they could both benefit from a symbiosis.
Magnus