Would it be possible to hack Maemo to make it a multi-user environment?

After all, Maemo is Linux and it comes with all the tools for multi-user access control that are standard in linux. By default, there is just one interactive user "user". However, I'm sure you can modify the configuration files and add other users with a different home directory.

It can work like this: Each additional user get its own home directory. They get read permissions to "opt" and a symbolic link from their directory, and read+write permissions to a specific directory within MyDocs. They do not get permissions to install any new applications. They can run any existing application, but it would use different data (different phonebook, conversations etc.).

You can have an app for switching users, requiring a password. After using it, the user can choose which user to login to after the next restart.

Just an idea.