I have answered this question many times now on the Technet forum so I think it is time to write about it here as well.
When installing Windows 7 from the DVD manually the partition needed for Bitlocker is created automatically, when deploying a captured image using SCCM however this has to be created manually.
The easiest way to create the partition required by Bitlocker during OS Deployment is to use the following command in the task sequence.
“BdeHdCfg -target default -quiet”
It will shrink the System partition and create the partition needed for bitlocker. This command must be run when the Windows 7 is installed.
It can also be run after OSD if the decision to use Bitlocker in the organisation is taken after the deployment and roll out of Windows 7 is complete.
If you are deploying Windows 7 X64 the “Disable 64-bit file system redirection” must be checked otherwise it will not work.
A reboot is required after the command is executed to complete the creation process.