About a year ago Microsoft released the BitLocker Drive Preparation tool to help with the disk partition changes to support BitLocker.  Information on using this tools is located at http://support.microsoft.com/kb/933246.  However it hasn’t been updated to work with Vista SP2.  If you try to install it on SP2 you get an error indicating it doesn’t apply to the installed OS.  Until Microsoft updates the tool, you’ll need to install it before installing Vista SP2.