If they do, they didn't bother letting other parts of the OS team know about it, or the law enforcement agencies that would be looking into was around it; probably at the request of the DOD and other government agencies that wanted to be able to use it without worrying about something as obvious to exploit as a back door.

And nothing (except system speed) stops you from installing truecrypt on top of bitlocker if you
really want to be paranoid.