With the phone, it’s natural to have one user associated with the device, logged into email, facebook, twitter, linkedin, instagram… need I go on? It’s a one user device.
On the tablet, Android JellyBean has started to give you the ability to have multiple users. I can leave my Nexus7 around the house, and as long as I have set up users for my wife and daughter, they can use it too. I have also created a guest account, however it’s pretty useless without a Google account associated with it, so you might as well not bother.
When the tablet is idle – that is to say sort of “logged off”, which means at the login screen, email (and FB, etc) notifications for the primary user appear in the notification bar. The notification alert sounds when email arrives for the primary user. The sender and subject appear for a short time.
The “password” I currently use is a swipe pattern. Anyone can see me use it and remember it. It is the only thing protecting me from their ability to get into my email etc and mess up my entire online life. Great if you trust them, not so much if you don’t.
I’m not even certain that the multitasking features of Android partition the users from each other.
In short, there is a long way to go before this is really worthy of multiple user operation in practice.
As for Apple/iOS? I no longer use them but as far as I know, they’re even further behind in this respect.