I have been working on the CCK 6.x port to rework the widgets to use FAPI. In the process I ran into a number of things that confused me, so I wrote up a tutorial on how to create custom elements in 6.x (at http://drupal.org/node/169815). It includes information on the new #element_validate and #value_callback arguments, as well as how to use hook_elements(), #process, #validate, and #submit, etc., and tries to identify what order things happen in and exactly what information is available at each step in the process. I would love to have anyone else familiar with FAPI check to be sure nothing has been misstated, or add anything that might make it clearer or more useful. It kind of goes along with the forms workflow illustration at http://drupal.org/node/165104. Karen