Look at b2evolution?
Check out [link|http://z.iwethey.org/forums/render/content/show?contentid=191359|this thread]. FuManChu was doing something similar. It [link|http://www.amorhq.net/blogs/index.php/fumanchu|looks like] he settled on [link|http://b2evolution.net/|b2evolution] for a multiuser blogging solution.
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Steve