Miles, URL links and mailto entries do not require that the whole email message be sent as an HTML document. I'm not sending this as HTML, but the links in my .sig below still present as active links in most GUI email readers. I predict that market forces will eventually drive email in that direction. I'm thinking of a situation where a cruise line, say, may want to use subscriber-based lists to promote its product, with full color and graphics.
Today is an example of a highly produced graphical newsletter in a list environment.
The problem, it seems to me, is that one pipeline cannot accommodate all. Therefore, separation of pipelines is called for. I believe I've come across a few producers who already do that, like Russia Today. Compressed back issues are available for anonymous FTP from FTP.GreatCircle.