This is a big and important patch for Drupal's security.  We are switching
  to much stronger password hashes that are also compatible with the Portable
  PHP password hashing framework.

  The new password hashes defeat a number of attacks, including:

  - The ability to try candidate passwords against multiple hashes at once.
  - The ability to use pre-hashed lists of candidate passwords.
  - The ability to determine whether two users have the same (or different)
    password without actually having to guess one of the passwords.

  Also implemented a pluggable password hashing API (similar to how an alternate
  cache mechanism can be used) to allow developers to readily substitute an
  alternative hashing and authentication scheme.

  Thanks all!