What you need to understand is that “Tags” are not a Livejournal feature. They are a Technorati feature that Livejournal has just, according to your news piece, implemented.
Technorati already reads WordPress categories as Tags. There’s no need for anything new.
If your blog software supports categories and RSS/Atom feeds (like Movable Type, WordPress, TypePad, Blogware, Radio), just use the included category system and make sure you are publishing RSS/Atom feeds and your categories will be read as tags.