As an alternative to the somewhat controversial idea of allowing users to select to place their signature above or below a reply (top-post/bottom-post), an even more useful tool would be to have a button on the compose page that inserts the signature at the current cursor location, a la Thunderbird's QuickText plugin. This would accommodate the top/bottom post issue, but also allow users to add a full, formal signature when needed and not on every short "me too" message.