Why do you imagine that informal collaboration isn't possible with offices? It happens all the time. Maybe you're just being prejudiced against a situation you have no experience with?

Because I've worked in both environments. I say informal because in open spaces you get involved in conversations and collaborative efforts that normally you have to go out of your way to be involved in. Here's a good example.

I was having a conversation with a dev about building an iPad app in my spare time and another dev sitting near by jumped in and argued that I should focus my efforts on the Mac App Store since it would be new and relatively easier to get noticed. 3 months later and I have an app selling well enough to quit my job if I really wanted to. This golden little side comment would not have happened if we all had offices.