Hi! I love the fact that more and more libraries are talking blogs these days. You might be interested in reading through the topic on
The Professional Blog.
I think it's a great idea to use a private internal blog to get staff used to the idea so that they will get why it's cool and effective to start a public-facing blog. It sounds like your use of a blog will be more to elicit ongoing conversation rather than soapbox-type broadcasting by one or two individuals. Message boards are also good for this kind of communication, but I don't know of any free and easy to use software for that.
Does anyone have a handy list of all the free blog options?