I want to upvote this one meeeeleeeon times :-(
Like many I suffer management that believe that 'communication' is key, so a big noisy open-plan office with a mix of coders and people who are on the phone all day is an excellent idea.
People took the piss when I started first wearing ear defenders but before long more and more sets of headphones etc were appearing around the office.
These days I just about survive with some Bose QC headphones. As mentioned elsewere its the nearby speech that's the real distraction for a lot of people. On their own the QC's make people sound 'further away' is the best way to describe it, they do work very well but can't block out the most determined oxygen thieves.
Last year I discovered mynoise.net - I suggest checking it out and perhaps bunging the developer a few quid in appreciation if you use it (no link to him, just find the site really good). Playing a mix of background noise generators in conjunction with the noise cancelling headphones is usually enough to totally block all the office chatter.
Now don't get me started on the window wars.... (for the record, I vote 'open' ;-)