I think I am not alone with disliking HTML mails in general (because they are massively abused nowadays, being a security risk and a vector for spam, phising, and viruses). The HTML mails Moodle sends out as copies to a forum post have been confusing some of our users (they didn't know what all these links representing the breadcrumbs are good for). IMO, the wisest decision is not to use / disable HTML mails at all.
Now that Moodle 1.8 will soon get official, there would be the chance to rethink its email policy and, at least, change the default mailformat from 1 (HTML) to 0 (plain text). Moodle calls the HTML mail format "Pretty HTML", where I find "Silly HTML" more appropriate
In addition, I am planning to write a patch that adds an additional config variable called 'enablehtmlmail' that has to be checked in order that users can prefer HTML instead of plain text. However, admins like me will keep that checkbox unchecked, to prevent users from making this bad choice.
best regards,
rob.
PS: did I say I don't like HTML mails?