As far as I know it automatically becomes attached to it; and if you change your avatar that will become part of the old posts as well. Justin knows the mechanics of this, but basically the "post" is generated new every time you click on it -- so the details of the post, the details of you (signature, avatar, number of posts, reputation, etc) are all assembled at the time a given post is accessed.

That's what I thought, too. But my current signature isn't displayed in posts I made before I changed it.