Totally, but the same will apply for what's e-mailed, because at the moment you can send an HTML e-mail of the invoice from the site. So in Drupal 6 you can call the same theme function that would display it on the site to add it to the body of an e-mail... alternately, you could just use the UI to create a fully text version of the invoice for e-mailing. Honestly, I'm way tired of the current invoice template and hope to see that change sometime soon.