This has been discussed many times. Personally I think it's very important to have some sort of user restriction, I do not see it as being the same as a phone. I do have to wonder if Apple is purposely leaving this out in order to sell more iPads. That would be unfortunate. As it is, it's very difficult (even potentially dangerous) to share an iPad around, especially to semi-strangers, which is a shame for a device that begs to be shared. Hopfully it's just something Apple hasn't gotten to yet (the initial software has a lot of rough edges and omissions, it was clearly rushed for launch) and they'll add it in a future update.