I remember reading a post where a developer mentioned adding masks and rulers to the tool as being a priority, but that they are focusing on developing a good way to implement it into the UI. Based on reading posts on the topic my best guess is that the implementation will be somewhat like this mock-up posted by a user:

http://www2.ambientdesign.com/forums/viewtopic.php?t=2493

If they do this it would be very cool, and users could create and share their own masks.