Android Police has discovered that Google has been adding bits and pieces of code to Android over the last year to get ready for multiple users sharing one Android device. Multi-user switching may be one of the key new features of the next Android version. Each user will have their application data and system settings saved in separate system folders.
Right now it is almost impossible for any tablet owner to share their tablet with someone else. When you log in to an application all your data gets synced and you don't have to sign in again. But when someone else wants to check their Facebook, for example, they have to either sign in in the web version, use another application or sign out the owner and sign in themselves.
The first hint to the user switching functionality was found in the lockscreen source code. A newly added method onUserChanged(int userId) will make sure that when a user switch happens the lockscreen is reloaded with the new user's specific security type. After being shown that method by one of the CyanogenMod developers, Ron Amadeo of Android Police started looking through the Android repository where he found numerous other changes that point to Google adding multi-user support.
One of the major commits to the source code was a commit titled "Plumbing in PackageManager and installd for multi-user support." done in April 2011. This commit meant that all application was no longer saved in /data/data, but instead in /data/user/0. This change is already visible on devices with recent Android versions. To retain compatibility /data/data now points to /data/user/0 (using a symlink).
(source: Android Police)
The second commit (titled "Multi-user - 1st major checkin") contained changes that hinted to a way to switch between users from the power menu, but this might be an early testing feature. Other commits included package restrictions per user, which basically means that you will be able to disable applications per-user if you don't use them. No UI work has been found so far.
Google has not announced anything about the next Android version or planned features, so it's unsure in what Android version this functionality will be made available. What we do know is that we're eagerly waiting to try it out!