If the messages are of a specific post type (assume “message”), you can display all these posts via a custom loop.
Some reading material to get you started – http://codex.wordpress.org/Post_Types
I would take custom post types if i ccould modify the visual editor to a plain text editor but it may don’t work. Is there another way to do this.
Greets,
MDickie
Do you already have data in microblog format? If so, how has that data been entered?
Somehow, I still feel that creating a custom post type and modifying the editor (maybe hide the entire editor and use the excerpt portion for input only) would work for you. I haven’t much experience with making such a change, but I found some links that (from the looks of it) seems easy
http://www.netmagazine.com/tutorials/create-wordpress-custom-post-type
http://sixrevisions.com/wordpress/wordpress-custom-post-types-guide/
Again, if you are not averse to learning from similar plugins, this one should give you a good sense of direction
http://wordpress.org/extend/plugins/simple-microblogging/