Get notified by notification centre when someone ssh's into my computer?
Applications like Growl or GeekTool may be able to help you with this. Growl can send messages to the Notification Center or use its own notifications; when coupled with an external system application like Log Surfer, it may be able to provide the notification you seek.

GeekTool would be much easier to setup, but doesn't send info to the Notification Center. Instead, you can have it display the output from a standard UNIX command on your desktop. In particular, you could have it display the output of a 'who' command, and update that every few seconds.