This is great - I've love to see somebody solve the concept of drawing constraints (personally I think the Interface Builder system, while comprehensive, leaves a lot to be desired).
I feel like the key to a good graphical constraints system is to come up with a consistent visual language for expressing constraints. I would expect to be able to toggle visual constraints on or off, or have several views into the same workspace with constraints on and off, at different sizes and aspect ratios.
Edit: By 'visual language', I have a very specific idea of what I mean - it should be rigidly defined enough to describe any combination of constraints, yet simple enough to be written down on paper in a design meeting.
This is amazing! I would love to be in the know about beta testing opportunities. Following you on Twitter now (@resilient). Awesome job!