Tomorrow’s KB935509 release is a little different than past updates. It is a prerequisite for installing SP1 and is applicable only for Windows Vista Ultimate and Enterprise users, because it only affects versions of Windows Vista supporting BitLocker Drive Encryption.
This update modifies the boot loader to enable successful install or uninstall of the service pack if the PC uses BitLocker encryption. We’re releasing it now to lay the groundwork for future updates and to eliminate any extra reboots by installing it along with updates that already require a reboot. For more information on the update, see KB article KB935509 (which will be posted on Tuesday morning).
In addition to KB985509, there are two other updates that will be prerequisites for Windows Vista SP1, for all versions of Windows Vista. They will be released at a later date and will improve the installation experience and reliability for the service pack as well as for individual update packages.