I'll answer your questions in turn

1. Is the only way to learn of new discussions in the Forums (here) to log in and read theBulletin Board? (I understand that I can ask for emails of replies to a topic I choose to follow, but how will I learn there is a brand new topic without logging in?)
Once you have registered and logged in a cookie will be set so to see if there is anything new you just need to visit theBulletin Board. If you wish to be Notified of any new Topics click on the Board name "General KM Discussion" and you will be presented with a list of the current topics, click on the "notify" button and you will be asked to confirm that you wish to be notified of any new topics. When someone starts one you will get an email.
2. Will the exchanges we had in the old group continue solely in the "Discussions" group, i.e. via email?
As far as I know Yes that will continue as before.
3. Will the topics from the Discussion group also be logged in the Bulletin Board, so if you come to read the Bulletin Board you will see *everything*?
No, this Bulletin Board is a separate feature, the Discussion list will remain the primary form of communication.
4. If not, what criteria are used to determine when a topic should be "moved" from the mailing list Discussion group to the "Bulletin Board" as a "side conversation"? How will people be able to receive subsequent messages on a thread that gets moved to the Bulletin Board...will they have to log in and manually ask to receive subsequent postings? (this sounds like a recipe for disaster, if it's true)
That's one for you the Moderators to determine. Most likely if a discussion is between two or more people and may not be suitable for a widspread brodcast to all members then it will be suggested it be moved to the Bulletin Board with a notice to everyone on the Discussion list of that fact. A very lively discussion can become very disjointed as people start quoting one another and responding at different times.
5. Will a digest furnished by the Bulletin Board include only Bulletin Board messages/threads or will it include the "Discussion" group messages as well? (Will we still get a Discussion group digest?)
No if you notices from the Bulletin Board they will only be from the Bulletin Board, the Discussion list is separate.
6. If we reply to an e-mail to respond to a topic, will our reply automatically be entered into the Discussions or Bulletin Board lists or both?
To reply on the Bulletin Board you must come and visit and type in your responce.
Again the Discussion list is separate.
7. Is is really necessary to have two places for one group? I think you risk losing readers/participants in the confusion (or maybe I'm the only one who is thinking there are two separate places to participate and they are not integrated).
Maybe that's one you you all to discuss?
The two are different ways of working, personally I like the Bulletin Board and have used them for years. Using a Bulletin Board requires much more time online where using a mailing list as you all currently do it can be read offline.
I'm still finding it confusing to know how to be sure that I remain plugged in to the actKM group. My personal preference would be to have:
* A single source for all messages/discussions. You can pick what to call it, but I will participate by email, and if you rely on me to log in on a web site somewhere be active, the reality is I won't remember to, despite good intentions! Sad
* Digest that will encompass all topics discussed by actKM members in both places in one digest
* The ability to reply to an e-mail (or create an email using a topic heading) and have it go out to the group, as well as be logged in the Bulletin Board or whatever place messages normally get catalogued and accessed.
That's fine and as things stand at the moment that's the way things are, you will all be using the Discussion list and send emails to
actkm@actkm.org which will then go out to everyone who has subscribed. This is no different to what you have been used to with the Yahoo group.
As an example, I have posted this message in the Bulletin Board. Did the "Discussion" group people get it e-mailed to them, or will they only see it by logging in to the Bulletin Board?
The only people who know about this are those who have visisted the Bulletin Board, you do not have to register to read the Bulletin Board but you must register to post. That said you have raised some interesting questions and I wil send an email to the Discussion group to let everyone else know what we have been talking about.
Sorry to be so obtuse...

Not a problem, Bulletin Board are obviously a new thing for you where I use them every day and discussion lists are something I am not that familiar with

[Mod - update changed "Forum" to "Bulletin Board" to clear up some of the confusion now we have renamed things]