I worked for a company that has an open-plan office and the work environment did not do anything to improve in-person communication between developers. I'm sure that it is possible for an open-plan office to be a positive thing, but by itself there is no added benefit. The reason that having an open-plan office made no difference was because everyone communicated via IRC. I would spend more time typing questions into IRC than I would spend talking to co-workers who were sitting right next to me with their headphones on. The fact that the company had an open-plan office did nothing to prevent IRC from being the main communication vehicle.

There were definitely positive things about IRC being the main communication tool, but it killed off any potential benefit that having an open-plan office would provide.