Just to throw another alternative into the mix... LISTSERV Maestro 
will let you define one mailing that contains both text and HTML 
parts and, based on subscriber preferences, you can have Maestro send 
just the text part to those who prefer text, and the HTML/text 
multipart to those who prefer HTML.

-- 
Francoise Becker <[log in to unmask]>

Knowledge is just a click away: http://www.lsoft.com/optin.html