There's a couple of similar sites that exist. They're lighter on the features of brainstorming, but I could see both easily expanding to have these types of features.

The first: http://www.startedby.com is specifically for throwing out ideas and starting to converse on them. It's very much for getting an idea out there and beginning to coordinate with others on it.

The second: http://www.builditwith.me is a bit larger and for the latter part of the idea stages. It seems the ideas are usually defined and its people looking to connect with the opposing skill set either designer or developer of what they need.

Obviously there's definitely a need, the hard part is how do you target and what features do you offer. Are you targeting a technical or non-technical crowd. Are you hoping to focus more on the idea process, or helping connect others that already have the idea but can't execute?