> on an os x laptop, you can be the logged-in user and read everyone's data
Shenanigans. Unless you have the password of the logged in user, you can't read stuff belonging to other users. Further, if the user in question does not have an Admin account, you're shit out of luck even if you do know their password.
Their home directory defaults to world readable, and the default umask is set so documents created are world readable, so you can read things in their home directories.
The Desktop, Documents, and so on directories are 700, so things in those should be unreadable.
if you have physical access you can do anything
If you have physical access to a machine, then you can almost always read the files off of it, except in certain cases with encrypted volumes.
on an os x laptop, you can be the logged-in user and read everyone's data. file permissions don't really mean much when everything of importance on the system is owned by one user (which is running dozens of applications with large attack surfaces).
that's not really a criticism of mac os, because it's the same on a windows desktop. you need elevated privileges on both systems to be able to do certain things to the system, but if all you want to do is steal sensitive documents, spy on the system's webcam, launch DDoS scripts, or add a command to the startup/login sequence, there's no need to bother elevating privileges.
one way to fix that problem is to make the system actually use the file permissions and user separation that the system already has, so that safari is running as a separate user with no access to the operating user's home directory, and that itunes has no access to the machine's webcam.
i haven't really looked into the sandbox feature of lion, but i'm assuming it does pretty much that, just like ios' concept of each 3rd party application being segregated from each other and not able to read files it's not supposed to.