As part of a project for a client, I needed to do a MIME multipart email which would include both a plain text and HTML version. The HTML version was supposed to be the default version. However, I discovered that the MIME specifaction is somwhat counter intuitive. You must put your prefered version as low as possible and list other versions in ascending order of prefernce. So in my case text/html went below text/plain. I might try to put on exmaple up to explain things better.
For some reason, I could not find this anywhere in my searches. You can get the permalink for the current post or page using:
get_permalink( $post->ID );
Of course this only works on posts or pages, not on searches and the like. I have no idea why I had such a hard time finding this.