You do not ever delete the topic, well unless you're the last one to remove yourself from the topic, more than one user belong to every topic so it remains in the database for the other users.
By clicking "remove myself from this topic" from wihthin the pm is the same as clicking "Delete" on the main pm listing.
If you, rick, myself, and jgeoff for example all have the same pm, clicking "delete" from outside will remove you from the topic, and leave the topic in place for rick, myself, and jgeoff; this is the same for if you where to click the "remove myself from this topic" button.
