We have the best of both worlds; html and text in the same email. This is done by using a multi-part format. Our email newsletter service provider, iContact, works the magic in the code they generate. Somewhere at the top of the message they put:

Content-Type: multipart/alternative;
boundary="???????"

and do some other stuff. We produce the email in both html and text versions. The email client renders html if the client is set to read html, and renders text otherwise.

Les Allan
Director
Business Performance Pty Ltd
www.businessperform.com