One of the things we did here was insure that managers stayed "in the middle" since at a small company its important to be accessible. That being said, sometimes as a manager one has to look at data which is private (say one of your direct report's current compensation plan) and for that we have private offices where folks can take their laptop and work privately for a while. Its a reasonable mix.
Its interesting to see some of the changes that have impacted offices, and phones and IRC are a good example. It used to be that a company phone meant a unit on a desk somewhere, these days it can easily be a cell phone. With that restriction lifted.
I find the tools (workstations, laptops) vs file cabinets, notebooks, and reference books, makes it easer to be 'present' where you have your laptop and your phone. And while 'open plan' is less expensive (certainly from a build out perspective) the communications you miss can be a bigger cost over all.