Multi-user support is one of the few remaining things a desktop OS can do that Android can't. The "coffee table tablet" use case would greatly benefit from a multi-user setup, as would an enterprise user who wants to keep work and home separate. It's been a top 20 item on the Android bug tracker since the debut of Honeycomb, so there is certainly demand for it.
As we've seen from my previous experiments in sticking my nose where it doesn't belong, Google likes to leave breadcrumbs in shipping products for the astute observer to find, and the multi-user situation is no different. After a bit of research, I can tell you that Google is listening. There is a surprising amount of multi-user work being done on Android - some of it is even working on devices right now.
Before we jump into things, we're going to need to learn some vocabulary. "AOSP" is Android Open Source Project, you should know that one. It's the publicly available code base for Android. A "commit" is a code push to this code repository. You're supposed to include a helpful comment with your commit, telling people what the new code is supposed to accomplish. This is Android code, so the commit comments are written by Googlers. In other words: they're accurate.
Now, on to the evidence:
So, in my very-much-non-expert-opinion, it looks like the following things are separated for multiple users: lock screens, installed applications, running applications, application data, default applications, home screen widgets, accounts, syncing, and language.
And the following things still need to be worked on: Namely, every piece of UI. You need a user switching screen, an entire settings section for managing users, setting permissions, taking or picking user avatars, names, etc. You'll also need a way to know what user is currently logged in. There also doesn't appear to be separate settings for home screen shortcuts, and probably a million other settings. And who knows how 3rd party apps would really deal with a massive change like this. Lots of testing needs to happen.
The first commit was over a year ago, and the last one we know about was 4 months ago. A lot of work has already gone into multiple user account support, but I'm sure there's still a lot more work to be done. Keep plugging away Googlers! I'll be watching...