We've rescheduled Drupal 8 beta 12 for June 29, 2015 to provide a little more leeway time for Drupal 8 core issues that require an update function. Starting June 29, any Drupal 8 core issue that includes a data model change must include an update function and update path test. See the previously announced core policy on requiring hook_update_N() for more information.
Identify your data model changes now!
If you have any core patches that are currently under development, please start identifying the data model changes in those patches now. Tag any issue that requires a data model change with D8 upgrade path and document the specific data model changes in the issue summary. Or, help triage major issues with specific attention to the data model changes.
It's actually also a good idea to start writing hook_update_N() implementations for your patches now! This will ensure in-progress issues can be committed as soon as possible following the next beta, and if your patch is ready before June 29, your update function will still help provide the corresponding update for the head2head project. Plus, if you uncover a critical limitation with Drupal 8 core's update functionality while working on your update function during the next week, the upcoming D8 Accelerate critical issues sprint will be an opportunity to get that critical issue fixed and unblock your patch.
Note for Drupal site owners and developers
Betas are good testing targets for developers and site builders who are comfortable reporting (and where possible, fixing) their own bugs. Betas are not supported releases of Drupal, and generally are not recommended for non-technical users, nor for production websites. The current beta release includes known critical bugs, including publicly disclosed security vulnerabilities. More information on beta releases.
The addition of update functions for Drupal 8.0.x issues should make it easier for developers to update their existing development sites between beta releases using update.php. However, these update functions may include bugs or even introduce data integrity issues, so developers should always back up the site and be prepared to rebuild it from scratch or manually update or migrate the data in case update.php fails to update it properly.