In addition to the options for updating Acrobat 9.0 to Acrobat 9.1 mentioned in my previous post, it's now also possible to use the auto-update feature within Acrobat 9 to download and install the patch.
Launch Acrobat 9 for Windows or Macintosh, then go to Help > Check for Updates.
Reader 9.1 requires a full download of the updated version; there apparently is no patch available.
For additional information on issues resolved by this update, read the full release notes. Since the new version addresses a critical security vulnerability, all users are urged to update as soon as possible.