I ran across an old locked thread on this forum while searching around, and figured I could provide some updated information. I have written a script for the Greasemonkey Firefox add-on that allows you to automatically a different HTML signature for each email address you have set up in your gmail account.

http://blankcanvasweb.com/gmail2_html_sigs/

Features:
* Works for Compose Mail, Reply, and Forward
* Support for single or multiple email addresses
* Create a different signature for each email address you use in your gmail account
* Signature automatically inserted into message above quoted text
* Signature is automatically replaced when you change the address you are sending from
* Easy setup interface

Aren't mail signatures so pre-dub-dub-dub....? Funny for the first week of one's initial "Internet presence," an irritating (and here RECURRING) reminder for recipient of the unreflecting oversize ego of the sender thereafter.