Open floor plans have been a common thread in my first- and second-hand startup experiences. However, I think this is more due to startups not spending money on office space than anything else. As a startup grows beyond a core group of hackers, the open floor plan stops being a nice thing and starts becoming a hindrance to productivity.

I like the idea of private offices that are shared among 2-3 people, as long as there is also ample lounging/social space. If the latter is not available, then open floor plan with headphones is preferable.