the only way i've been able to figure out how to do it, is to just attach a normal .html file to the email.
this isn't exactly ideal, since you can't specify separate content types (so if someone wants plain text instead of html email, they won't get it). and it won't work with every email client (like entourage, i believe). but it will work with apple mail and webmail.
some things to keep in mind if you go this route:
you can use style sheets that will show up in webmail, but not in clients (like apple mail). so if you specify a style, also use a <.font color="" size=""> tag to cover all bases.
do not include any text in the email, just the .html attachment (or this way won't work).
remember to use full paths to images and style sheets (ie <.img src='http://myserver/img.jpg'>
hope this helps...