Good general linux question and a somewhat philosophical question. There actually was a pretty good article on slashdot today about this as it relates to Mac OS X.

The way you control access to to resources on your system is via users and groups. There is no way I know of to have everyone log in using the same user name but give different people different privilege levels. You can force sudo to use a password and set it different than user.

But this is a wireless tablet PC, not a desktop or a server. Why would you be giving other people access to your system via ssh? If you really need a multi-user environment this is really an awkward platform for such a thing. But this is Linux, you don't need a good reason. You could set the system up as a file server if you wanted.
David