Howtogeek posted a guide about running Windows 8 Developer Preview from USB drive

Running Windows 8 from a USB should not be confused with installing Windows on a USB drivein this case, instead of installing it on the drive, were just running it straight from the portable drive. Heres how to do it. This is actually a new feature in Windows 8 called Windows To Go, but if you have been using the Windows 8 Developer Preview you will know that the binaries were removed for this release. The feature which will use a wizard called the Portable Workspace creator relies on existing Windows Deployment technologies, most notably imagex.exe which is a tool used to manage .WIM files. In order to do this there is a few things we need: A bootable Windows 8 PC (You can also use a Virtual Machine) A USB 3.0 16GB USB (This will work on a USB 2.0 but might appear to be a bit slow) A Windows 8 ISO File or DVD An active internet connection (To download the WAIK) So grab those few things and a hot cup of coffee and lets get started, at this point you should have your Windows 8 PC booted up and your USB connected.

  How to Run Windows 8 Developer Preview From a USB