We try to notify this list when API changes are made in Drupal 7, so contrib and custom developers have a chance to fix their code before they have the pain of figuring out why it's broken.

1. (issue) Several Search APIs changed: 

• search_data() return value has changed to be a renderable array rather than rendered HTML
• hook_search_page() return value has changed to be a renderable array rather than rendered HTML
• theme('search_result') and theme('search_results') and their templates/processing functions now use a 'module' argument/variable in place of 'type'

2. (issue) drupal_placeholder() now takes a string as an argument instead of an array.

Thanks,
-Randy