The early copy of Windows 8 that was leaked earlier this week – which turned out to be real – gave an early glimpse at a feature called "Portable Workspace Creator."
The feature is described as:
"Portable Workspace is a Windows feature that allows you to run Windows from a USB storage device."
Those wishing to make a Portable Workspace will need a USB stick with at lest 16 GB free.
It also lists a requirement of having access to Windows 8 Enterprise Edition installation files, so this could be a feature made for business IT and not just for personal users. Perhaps Microsoft is restricting it somehow over concerns of casual piracy.
Microsoft hasn't officially talked about this new feature at all, but hopefully it will detail Portable Workspace, as well as other features like History Vault, soon.