I've long noticed when replying to emails within Gmail, that my signature is way down at the bottom of the reply below the original message so the sender probably never sees it. I sometimes would even cut and paste it to the top of the message.
What this tweak does is allows a user to go into the General settings and change it so that the signature shows up at the top of the reply.
Besides my Gmail account, I also have a domain-based email account hosted by Google Apps. The option does not appear in my Google Apps settings.