I would also love to help build a web app for a site like this. I tried to email you, but there is no contact info in your profile. Shoot me an email (in my profile) if you are interested!
I would love this site if it existed. I also have a list of projects that I would love to work on, but I like working with other people the most so I haven't started many of them. This site would also be a great place to meet new people with similar interests.
It would probably be difficult to go from a joint "proof-of-concept" project to a company unless all parties are on the same page. Like you mention, maybe the site could have optional legal agreements or something. Perhaps you could partner with something like LegalZoom.com to take care of those agreements for the users.
I used gigablast for a couple of searches and it just didn't give me the results I was looking for. I think I will stick with google until I find something that is consistantly more reliable