Last month, multiple user accounts were shown off in Android 4.1 Jelly Bean and while isn't likely to come to previous versions of Android (specifically Ice Cream Sandwich), it'll come to future versions. Multiple user accounts are hidden in Jelly Bean, but isn't fully working yet.
The Android team went on to confirm this feature and that they're working on it and will definitely change in the future. It seems that Android is not only getting fully functioning multiple user accounts, but also an innovative way of accessing those accounts. Yesterday, Google was granted a multiple user face-to-unlock patent.
If you recall, Google first showed off the face-to-unlock functionality last year when they debuted the Galaxy Nexus. Now, face-to-unlock isn't anything new and has been around for years, but hasn't made its way to mobile phones and tablets quite yet. Clearly, the Android team is wanting to integrate this functionality with multiple user accounts which we should see fully functioning in Key Lime Pie, the next version of Android.
With virtually every smartphone and tablet, they have a front-facing camera so to pull this feature off isn't too difficult. Especially with tablets and families, several people can share the same device so having multiple user accounts is a nice value add. Combine that with using the front-facing camera to see your face to unlock/log-in to your own account, and that's pretty awesome.