The only advice on HTML in e-mail should be: don't!
I suppose you could generalize this by saying HTML email is useful when you have images that are informational, not just decorative.
That said, maintaining 2 templates or deciding exactly how to handle the formatting of the text version can be a hassle. I'm curious how others have handled this problem.
At PostageApp our solution to the problem is a templating engine where you can write your HTML/CSS separately and we inline it at run time to ensure client compatibility. We then provide a separate Plaintext tab with an 'import from html' function which does most of the work for you for managing dual formats.