We currently notify unregistered users with an @ message on Twitter when someone likes their profile, or sends them a message or a smile. We only ever send one

We currently notify unregistered users with an @ message on Twitter when someone likes their profile, or sends them a message or a smile. We only ever send one of these messages, and never again, if they don't join after that. (See http://twitter.com/PlentyOfTwps http://twitter.com/PlentyOfTwps for examples.) When the user is registered, we contact them via email.