Several sites are reporting on a recently published patent application from Apple which details the ability to store user accounts on external devices (such as the iPod) and moved/used on another computer.
Hence, by coupling the external, portable data store to another multi-user computer, a user is able to login to any supporting multi-user computer and be presented with their user configuration and user directory.
This idea is not new and was first revealed by Apple in 2003 as they were preparing for the Mac OS X 10.3 (Panther) release. "Home on iPod" was a feature that was inadvertently posted on their website with the following description:
Home away from home
Ever thought you could carry your home in the palm of your hands or in your pocket? You can. Panther's Home on iPod feature lets you store your home directory - files, folders, apps - on your iPod (or any FireWire hard drive) and take it with you wherever you go. When you find yourself near a Panther-equipped Mac, just plug in the iPod, log in, and you're "home," no matter where you happen to be. And when you return to your home computer, you can synchronize any changes you've made to your files by using File Sync, which automatically updates offline changes to your home directory.
The description was quickly removed by Apple, and there has been no further mention of the feature.