I set the document so that it can be read without logging in, but you must log in to edit it. Almost as soon as I sent the invitation to collaborate, a colleague logged in and started editing too (thanks Anna!)
Unfortunately I can't see what my colleague is doing while she's doing it, so it's still possible for us to be editing the same section at once. Makes me wonder what ever happened with Google Wave, and when they'll integrate that real-time collaborative functionality with docs? I do find Google Docs handy, but unfortunately it doesn't handle other word formats very well. I use it as emergency back-up, but wouldn't keep rely on it for final versions of work.
I never knew about the survey options in Google Docs though, so that was new, thanks Isla! I just set up a basic question to see how it works: