Earlier today, someone decided to post to the Android issue tracker complaining about the lack of multiuser support for smartphones. Within a few hours, a developer at Google responded and closed the issue, remarking that "the development team has implemented this feature and it will be available as a part of the next public build." Sounds pretty definitive to us.
The "next public build" is the only ambiguous part of this statement, though that Googler is almost definitely referring to the "L" release of Android scheduled to land some time later this year. Multiuser support on smartphones is something Google had indicated an interest in previously, but it limited the feature to tablets as part of Android 4.2. At the time, multiuser mode wasn't very configurable and tended to impact device performance somewhat severely, though it was significantly improved in Android 4.3.
This new functionality will likely come to the excitement of a great many Android phone owners with children, nosy friends and relatives, or who use their phones for business purposes.
I'd say the single greatest use for this will probably be in the workplace. While enterprise-managed and sandboxed profiles will apparently be a thing in Android L, most small and medium-sized businesses probably aren't interested or savvy enough to really dig into this kind of deep, high-security, high-complexity functionality. Putting a separate, password-protected profile on your phone for business email and documents will allow greater work-life separation for those of us who only carry one smartphone for home and business. After all, in this hyper-connected world, we could all use a way to put up at least a semblance of a wall between our personal and professional lives.