Yep drunk too much coffe today...
There is the (non working since not updated...) "groupblog" plugin, what would be interesting is the exact... oposite : allow any blog user (author ?) to publish his post (an excerpt) in the goups he is part of, as a nice litle checkbox option on the edit windows :
Publish this post in my groups :
publish on the wall
Why ? Because the activity wall of buddypress is just... well... not really usable as is. Too much things, all is lost....
Just a raccoon for this one