@Ovidiu: The only things I changed in it's behavior was moving the menus to Site Admin (so they are only accessible by siteadmins) and changed the database table and plugin options to store site wide instead of per blog. I also had to move around some files and code so that it would work in the mu-plugins folder. In it's current state, bad bahaviour isn't feasible for multisite for all the reasons mentioned already, so hopefully the author will make the needed updates for it. Use my modified version for the time being unless you don't mind your bloggers managing it's options, not knowing sitewide stats, and not being able to use http:BL without adding the key to every blog in your network - oh and "bad bahaviour statistics in the blog footer" is enabled by default so it's displayed in the footer of all the blogs in your multisite. The main reason I made the mod was because it creates a database table for every blog, and for a blog network like mine with more than 10,000 blogs.. that's a lot of unneeded database tables.
You'll know BB is active on all your blogs by looking at the BB logs, but as long as the menus are showing up under Site Admin you'll know it's working. Since it's in the mu-plugins folder, there's no need to "activate network wide" and it won't show up in the plugins menu.