I have this in my Task Sequence and it used to work but appears to not be working now, the reason I am noticing now is because when after MBAM Client Installs the Start Encryptiong which used to take 5 minutes or so is doing a full encryption there so to me Pre-Provision Bitlocker Task is no longer working... This appeared to start after upgrade to SCCM 2012 R2 SP1 and is not resolved with CU1.

My Task Command is:

manage-bde -on C: -UsedSpaceOnly -encryptionmethod aes256

So how did you solve it? That might be helpful for others that are running in the sam