I've put together a quick port of Jim's Modern Bubbling theme for Adium for Kiwi (a new theme-able twitter client, with rules!).
It's actually a complete rewrite, with some changes, but I tried to remain fairly close to the aesthetic.
I've added a hook for "muting" tweets: In the rules tab, choose "Add a CSS class" for the action, and type in "mute" (sans quotes). This will take those tweets that match down to about 30% opacity.