I've started work on updating the theming guide for Drupal 7. So far all I've done is rename the "Drupal 6 Theme Guide" section to "Theming Drupal 6 and 7" (and renamed the D5 part to "Theming Drupal 5") and made a few minor changes. What I'm hoping is that we can just add notes to the existing D6 content rather than fork everything.
One of the big chores will be to go through the 40-some items on the d6/7 theme update page http://drupal.org/update/theme/6/7 and make sure they've all been reflected in the documentation. Since there are so many items on this page, I was thinking we should make discrete notes right on that page so we don't duplicate efforts. For example, on the one bit I've addressed (http://drupal.org/update/theme/6/7#specify-all-css-js) I've added a note at the end like this: [docs updated]. Hopefully that will help other people to know what's been done and what hasn't.