Wow, there has been some amazing usability improvements going on in Drupal 6 lately. "Drag and Drop has been approved for the Blocks page":http://drupal.org/node/181066, and Nate has two more issues in progress to get drag and drop into "Menus":http://drupal.org/node/181126 and "Book":http://drupal.org/node/192736
I have been so inspired by this work that I have turned the work I have been doing with modal dialogs/popups into an patch and issue. Check out "Adding Modal Dialogs to Blocks Admin":http://drupal.org/node/193311. It has basic dialog infrastructure and two very cool examples of how they could be used.