I think what Ulrich is most worried about is he doesn't want a stable encyclopedia to be a fork. This can be easily avoid by puting a link to the current Wikipedia article on hte Nupedia version, labeled "Edit the latest version of this article".
Actually, that's not at all what I'm worried about. ;-)
What I'm most worried about is that Wikipedia will lose contributors, because most of them won't get a choice to decide what's in the "real" encyclopedia and therefore lose interest. The closer (the more "officially") Nupedia is associated with Wikipedia, the less interesting it will be to spend time for Wikipedia, because everything one is doing there is just the "foreplay" for the "real thing" Nupedia.
I admit that I have my personal problems with the Nupedia editing policy, and the way authors are treated there as more or less irrelevant (because the important persons aren't the authors, but the almighty editors). I once offered two ready 2 page articles on "Sauna" and "Sherry" for the Nupedia project. I got an awnser like this (I'm exaggerating here, to make my point clear): "It's nice, thank you, but can you please strip it down to one paragraph, then we will consider to put it in the editing process as soon as we've found an editor suitable to the subject". Guess what? I never thought of Nupedia any more, and I never heard of them, too. I doubt if Nupedia (if revived and taking [[Sauna]] and [[Sherry]] from the Wikipedia will ever be able to find someone (or even a group of people!) with a university degree in either Saunology or Sherryology that is willing to work for free and will "approve" those two articles.
But now back to the subject: Imagine what would happen if the title page of Wikipedia wouldn't say any longer: "Welcome to Wikipedia, the free encyclopaedia", but instead:
"Welcome to Wikipedia, here you can upload articles that probably will go into the free Nupedia encyclopaedia editing process some day if the people there like it and have the right approved editors for your subject".
Again - If Nupedia wants to use Wikipedia articles (forking them or not) - they're free to do it, the License perfectly allows for that. Just go ahead and do it - there will be enough Wikipedia editors merging back the approved articles to Wikipedia, both will profit from that. (However I'd like to see how that's going to work with that silly GNU FDL requesting to cite all the history. Merging a history from Nupedia and one from Wikipedia in the Wikipedia talk page (where else?) must be fun!)
But I strongly oppose the Wikipedia *supporting* that in any further way. It's like shooting ourselfs in the knee (is that a valid phrase in english?). Apart from that, there are some problems at Wikipedia that should be adressed by developers first (performance, searching, just two name a few.) I don't see a reason why one should spend development time to make Wikipedia suit for the Nupedia editing process.
Uli