Several other reasons why the email will show up as plain text.
OK, so you have set the default delivery mime type to HTML.
BUT, if a customer registers, and (on the registration page) selects PLAIN TEXT as the preferred e-mail option, then that will over-ride any setting you have made in admin.
Secondly, if the recipient sets their e-mail client to view messages as plain text, then there's nothing you can do about it. That's why ZC will always make the two options available.
If you have set up a TEST customer, login to your shop as that test customer and go to the "My Account" page. There you will be able to re-select the e-mail format. Change it th HTML.
Then, send a NEW email to your test customer (the old ones will remain TXT).
This time, you should see it in html.