How to restrict the level of privileges of applications on jailbroken devices?
Someone would have to write a sandbox for jailbroken programs, which would be extremely ironic, because the point of jailbreaking was to escape the sandbox. Normal apps still run under the mobile user when you're jailbroken, though, so there's nothing to worry about there. All jailbreak packages have full access to anything, and there's currently nothing you can do about that. Just play it safe, and only install trusted packages from reputable developers/repositories. A related question that I answered on the same subject can be found here.