Nate has some interesting and accurate thoughts on the academic states of blog posts. Roy had some interesting thoughts to add as well.
As for myself I think that there is little we can do to encourage bloggers to post meaningful content because it is their own content. That is the beauty of blogs. We control only what we subscribe to. If I deem the content of a blog to be frivolous I am free to not pay attention to it. Anyone who wants to join in serious discourse will already be motivated to ensure that they are posting worthwile thoughts. Nate has argued, rightly I think, that having high qulaity discussions will attract wider participation and will elevate the status of blogging in the academic community. The key is that it is an individual choice.
There will always be people who blog recreationally. It is no different than any other media, we have professional journals and then we have non-academic publicaitons. We are responsible to be professional if professionalism is the purpose behind our blogging. We do not need to worry about those who have no professional objective to their blogging because, in the end, a good idea is a good idea regardless of its source. Those who wish to put serious content on their blogs must trust that it will be recognized as such.