Here is the announcement of the participation of WordPress and Drupal.

Joomla has created a page on their Wiki which lists several ideas for projects to be done inside the program of the Google Summer of Code, for example:

• Advanced Poll / Survey Suite for replacing the current component for the management of surveys, no longer suitable
• Full Frontend Editing, add more functionality nell'editing Site via frontend
• Taxonomy Extension for better management of taxonomy
• Joomla Multisites to manage multiple sites at once
• SEF URLs with Sub-domains, such as to allow the creation of subdomains for some components
• Authentication Systems improved management of authentication
• .. and much more!

Obviously, these projects are experimental, but some of them will find a future as inclusion in the CMS or as a standalone extension.