I like all the new stack forums, I think they're like the fora equivalents of Twitter.
OnStartups seems to be a place where you can ask general questions about start ups, but is there a forum online where you can let others know about your idea and actually start collaborating together?
I think that the easiest area to collaborate is the marketing plan.
I say that startups will send us the Executive summary of their marketing plans and we will all help them in finding channels and tell them what worked for us.
I think this is the right forum to do it. Let me know if I got it wrong.
If you go to the top navigation, you will see a link for "chat". I think what you have in mind would be perfect for that. Keep in mind, you need 20 reputation points to use it. That is easy enough: just fill in your profile.
I would like to offer an alternative to @ron M.'s very valid position. We are doing a very public reworking of our company that we are documenting as we go in our blog. All this concern of secrecy is a holdover from the dot com bubble. It is all about execution these days. The fact is anyone can "steal" anyone's idea at any time. Getting it done in a fashion that the buying public likes is where the rubber meets the road.
There is a very old Chinese board game called Go. In go, it is the common practice of expert players to tell their opponents the exact strategy they are using to win as they play. A truly great player wins with complete transparency and no opponent can stop them. Business has to be like that, especially online business because you can't hide what you are doing.
This forum is called: Dropbox, e-mail, secure wiki, secure file system, secure source control. Startups are usually a secret. You don't want to share your progress and ideas with the world, only with those you work with -- you use dedicated tools for that, not "forums".
a forum may be a place to "meet" someone to work with- but very quickly, the interaction has to be diverted away from the forum.