Users would accept the fact that edit conflicts might arise. I accept it anyway when I'm making a large edit (e.g. a massive article expansion) and for those users who edit offline (there are many), they certainly accept it too. I think the point here is that, people make huge edits anyway, and this would only make it easier. Especially if a "Finalise" feature could also be implemented in the draft page.
If this could be implemented now, using JavaScript, I would like to see that (I'd certainly use it). It would possibly be better as opt-in anyway.
Another function this would provide is something like a manual AutoSave. If you're making a large edit the last thing you want to happen is for your browser to crash and lose it all (and yes I even use Firefox :P). It would be reassuring to be able to "Save" the page in progress without having to wait till you're completely done, and without disrupting the actual article.