I like the town hall analogy for bulletin boards. Not sure I like the blogs are public diaries analogy (IMHO blogs, in addition to being great diaries, are also great marketing tools, support tools, etc). My favourite definitions are still "blogs are digital paper" and "blogs are the unedited voice of a person".
I really like: "blogs are diaries that can be read by the public, while bulletin boards are town hall meetings in which the public can all discuss issues equally."
I think there's also a lot of insight in the ways that these two things can be used in conjunction with one another. The town hall quality of message boards can play off of the informational/journal quality of weblogs.