Why did my facet blocks disappear after upgrading to 7.x-1.0-rc1?

Due to the issue #1355270: Deltas starting with non alpha-numeric characters cause JavaScript issues, the Facet API delta hashing algorithm was modified to only produce alpha-numeric block deltas. If you are using the core Block interface to display blocks, running update.php should convert the block deltas to their new values and the blocks will reappear.

Note that there is no direct upgrade path for alternate block management tools such as Context or Panels, so site administrators will have to manually re-position facet blocks after upgrading to RC1.