"they can login if they have already registered."
registered for what? my blog specifically or any wordpress blog?
:), pardon the sarcasm, but you dont think you would have control over someone else's blog, right? Yes, your blog. Thats your admin area, it controls your blog. Not mine. :)
..my question has to do with future potential posters after i deselect "allow anyone to register", ones that haven't registered for my blog yet and now cannot.
Ive answered this. Future potential commenters that have not registered will need to comment anonymously. There are only 2 ways to solve the puzzle.
Maybe my lack of understanding has to do with what I see when I visit my blog on another computer (with anyone can register deselected). I see no way for a user to post unless they log in - there is no comment field that one can use to post anonymously.
If you do not see text fields allowing for someone to leave a comment in a place where you previously saw one for registered users, and the only option you have changed is the registration thing, then thats an issue with your theme, not the way wordpress handles comments. And yes, that would explain alot of your confusion :)