Many sites include a Twitter-style referencing system, where you can type @username to reply to someone's comment. When you do this, they are notified that you mentioned them with an email notification.

Perhaps this (or something like it) could be implemented at Wikidot?

e.g. The following post would notify Bryce when I posted it (unless he disabled such notifications in his account settings):

I really like [[user bcammo]]'s RoaringApps site - it's very well designed.

Something like [[*user username]] would also trigger a notification, and perhaps @username as well, but just mentioning their name (username) would not.

Activities could also include this information, with a new "Mentions" tab.