During the Drupal 7 development cycle, we made a couple of changes to theme functions, notably: 1. They now all take a standardized array of variables instead of an arbitrary list of parameters (see http://drupal.org/update/modules/6/7#theme_changes) . 2. When passing in a renderable element (such as forms, pages, content, etc.), you can pass in parameters via #properties (see http://drupal.org/update/modules/6/7#poundtheme) . Unfortunately, these two changes clash with one another in the case where you pass in a single non-renderable argument to your theme function, and we didn't catch this until the couple days or so before code freeze. It then took a few additional days to get an acceptable fix together. (http://drupal.org/node/600974 if you're curious about the backstory here.) Therefore, we have the first of what is *hopefully* a very short list ;) of Drupal 7 API changes post-code freeze. Please be advised that if your D7 module defines a hook_theme(), you need to change it in the following way: 1. Where you used to say 'arguments' => array('whatever' => NULL, 'whatever' => NULL), you now say 'variables' => array('whatever' => NULL, 'whatever' => NULL) 2. In the case where you're passing in a renderable element to your theme function (such as the case of a form theme function), instead of specifying it in the list of variables, you'll simply add: 'render element' => 'form' and omit the 'variables' line altogether. Sorry for the hassle, but believe me, this is a *much* better situation than not being able to have theme functions with one argument. :P And thanks so very much to those who've embraced #D7CX and have started porting your modules already! -Angie