Source Best explanation I could find so far.
It doesn't have BitLocker, but it does have its own form of device encryption, which is based on a Trusted Platform Module like BitLocker. That's not the same hardware TPM you find in Intel PCs today, it's part of the firmware in the system, but that's the same way that System on Chip x86 PCs running Windows 8 will implement the TPM, to keep power and hardware costs down.
The main difference between device encryption and BitLocker seems to be that BitLocker can be managed by an IT administrator in a business using group policies and a domain; with no group policies or domain support that won't work on Windows RT. If you can manage device encryption it will be through Exchange Active Sync – the way you sync email and calendar appointments – which can already make you use a strong password on a smartphone or a Windows 8 PC.
A little more detail here
Windows RT tablets will also have Trusted Platform Module (TPM) chips installed and thus will be able to take advantage of “virtual smart cards”. This is a new feature in Windows 8 that does away with the need for a physical smart cards (which users lose or forget) and smart card reader hardware. The credential stored in the TPM emulates an RSA authentication token so that the password to access the corporate network works only with that specific machine.
The RT edition of Windows also supports device encryption to protect company information that’s stored on the tablet, and instead of a regular password, which is subject to dictionary attacks or rainbow table hacks, Windows RT tablets (like all Windows 8 computers) can be configured to use picture passwords – whereby you have to touch specific areas of a photo in the right order to unlock the screen.
Looks more like hardware access Security than it does actual data encryption, sounds misleading to me.