Michael, as you suggested, the posts are still in the forums (phew!), so I won't worry too much about the links in the activity stream not working, as we're hoping to remove those and just have the list of forum posts going forward.
That brings me to a new question -- is there any way to remove the "Home" tab on a group? I'd like people to go straight into the forums (only for certain groups, which do not have any other applications associated with them except the forums). I know I can remove the Application Navigation widget, but then I lose the "New Post" button. Is there any way around this? For some reason, our Quick Post widget does not include Forums in it, so I need a workaround until I can get that part sorted out.