Go here: http://www.facebook.com/help/contact.php?show_form=delete_account
Click "Submit" and follow the instructions.
Your account will be deactivated for two weeks, and if you DO NOT USE FACEBOOK IN ANY WAY during that period, your account is permanently deleted.
Your profile isn't deleted right away! You must NOT log in to, or interact in any other way with Facebook for at least two weeks, as it will cancel the deletion request. That includes NOT logging in to Facebook using any client (like the iPhone app or IM:s like Pidgin), NOT clicking embedded Like-buttons on other websites, NOT logging in to other services using Facebook Connect (like Digg) etc etc.
While your're at it, delete all your browsing/forms history and cookies as well, to minimize the risk of accidentally using your account!
This method is official and should be complete, i.e. no need to delete individual photos, comments, messages or items from your profile or anywhere else on Facebook!
If you have any concerns whether your stuff is actually deleted (as opposed to being kept somewhere in the deep dungeons of Facbook's HQ) your best bet is to ask them using email: email@example.com.