There have been plenty of studies showing private offices increase developer productivity, and plenty of other studies showing the opposite. This is a hard thing to quantify, because, unlike the effect of a too-small hard disk, the effects are not immediate and obvious; productivity, whether it increases or decreases, will only do so over a long period of time.
I am here to offer my opinion on the subject: it does not matter.
Let me be honest: I don’t care whether it increases productivity or not. When searching for a job, I look at a variety of signals: salary, perks, health insurance, and yes, office layout. All these need to be weighed carefully: what’s better, $500k a year with horrible health insurance or $200k, no deductibles and free gourmet meals? The answer will vary quite a bit from individual to individual.
I have chosen to place a high weight on the office layout: I am (lucky enough to be) in a position to state categorically that I will not accept a software engineering position that requires me to work in an open office environment.
Productivity doesn’t factor into it at all.