After installing an trying out Proto, I'm impressed with this "start". It still needs a few things to truly be a great app though. Aside from the fact that you can't open a Proto file in Dreamweaver, although I believe this will be addressed shortly by an extension, I would like to see the ability to name div's that you add so that when the HTML is generated it is a little bit more meaningful than the random div names Proto generates. Also it would be good if you could enter div dimensions manually.
Adobe should also come up with a Proto viewer so that you can share with your customers.