To use BitLocker in Vista, you had to create a separate active drive partition. When Vista first shipped this had to be created manually, but Microsoft released a BitLocker Drive Preparation Tool later to help with the partitioning.
With Windows 7 this partition is still required but is created automatically when BitLocker is enabled. However the Drive Preparation Tool is still supported as a command line tool, intended mostly for scripting the BitLocker setup for multiple systems.
More information is available at http://technet.microsoft.com/en-us/library/dd875534%28WS.10%29.aspx