No idea how long this feature has been around, but six months ago I was not able to delete my account, all I could do was disable it. For some reason I went into my account last week, and was able to dig and found an area where you can request that Facebook delete your entire account and all data associated with it. It's a manual process, apparently, as you file a request. A few days later, I got an email saying the account had been deleted, and sure enough if I go back to Facebook I cannot log in.
So, for those concerned with Facebook's callous attitude toward privacy, this might put you at ease. If, that is, that they truly delete your data :) At this point I wouldn't put anything past Facebook.