Android 4.1 Jelly Bean brought with it an assortment of new features, but it looks like some unannounced multiple-user functionality is present in the OS as well. On the xda-developers forums, zanderman112 has posted a tutorial and video showing how to create a secondary account using a terminal emulator. The new account then shows up in the power button menu, allowing users to jump between the different accounts. The functionality looks to be rather limited at the moment, with the secondary account only having access to a small number of settings, and functions like text messaging reportedly showing up across both accounts. According to zanderman112, the method will work with any Jelly Bean ROM built from the Android Open Source Project.
Another forum member, cdesai, submitted a patch to the AOSP tweaking the order of the power button menu when the multi-user functionality was enabled, only to have the change rejected. In response to the submission, Google software engineer Amith Yamasani wrote that the feature is "not ready for deployment," warning that "Bad things will happen if you use it in its current state!" However, Yamasani also notes that the UI for the feature may change based on input from the Android user experience team, so it appears that the capability is something Google is planning to bring to future versions of Android.