If someone has allowed the Lightbox Registrations App to access their facebook timeline, then their timeline will be automatically updated with a message stating that they just signed up for the event.

You can specify all aspects of the message that gets posted to facebook. You can use the event logo, or a custom image. You can use the event website as the link, or specify an alternate. You can use the event description as the main message, or specify a different message, perhaps advertising for a sister-event, or offering something unique to other people who register.