Hey Bogdan, I just noticed I forgot another icon: pref-mention.png. I think http://img.wixardy.net/Bogdan/Preferences/Advanced/pref-mention.png is too Twitter-centric. George came up with this: http://img.wixardy.net/Current/Preferences/Advanced/pref-mention.png. Something involving a chat bubble and person/duck silhouette would fit nicely, I think, but I think his icon does not blend well with your set.
Cheers, Robbie
