I don't think user accounts work on the iPad. The hassle of having to log in at lockscreen, or having to log out every time would be a nightmare...
People already have unlock codes for phones, why not have a similar thing for the iPad. Then again... you don't
need to set a passcode, so the log in could be as simple as hitting your user picture. A question is then... how do you log out? Maybe it could be another option on the shutdown screen (holding down the power button).
Although, the idea of fast user switching in desktops (i.e. doesn't log the other user out) would be nice to see on tablets too. The only problem with that is how the iPad doesn't really have enough memory to support multiple user instances at once.