Hopefully it will be available on phones, as a 'Guest' mode would be perfect for lending your phone to someone to make a quick call or text message.

Nope :-(

"Available only on tablets.", see http://www.android.com/whatsnew/

Well that's disappointing. I suppose they thought the UI was too confusing to put it on smaller screens?

They probably also thought (understandably?) that the use-case for a tablet is more likely to involve sharing than a phone.

The UI is on the lock screen, so plenty of room. More likely (1) Honeycomb style rush job and (2) "everyone should have their oen phone"

Well, it makes a little less sense on phones, since unless you switch the SIM card, you are still treated as the other person as far as the phone network is concerned. It would be a little odd to be logged in as one person, but the phone network treat you as the other. And since there are features like importing and exporting contacts from the SIM card, those would be more complicated in multi-user scenarios.

I think there's a certain sense that a phone is a much more "personal" device than a tablet. While it would probably be possible to add multi-user support, it would be more difficult, and be less useful, than on a tablet.

While I personally find it annoying, I know many parents that hand their phone to their young children to play games or music. Multiuser support is very much in demand on phones, even if it offered nothing more than a safe guest account.

With kid's corner feature in windows phone 8, you can have fine grain control over what the kids have access to.

Either there's some hardware security thing (equiv to ctrl-alt-del interrupt on windows) that is needed, or perhaps they are wary of rolling it out as a universal feature (could be confusing for phone users who update)?

It's already being exposed in CM anyway.

I could even do with a guest mode that would allow the poor fellow to browse the internet (only on WiFi, even)... :(

I believe the recent Nexus 7 update was 4.1.2 and not 4.2, so this feature is (unfortunately) still missing for Nexus 7, as others have affirmed.

4.2 is officially due on November 13. We shouldn't really be expecting it before that.

Although it is a Nexus device (And a new one) so I'm sure it will be updated soon.