1. A computing device for use with multiple user-level execution environments wherein the environments are isolated from one another to simultaneously protect personal privacy and enterprise security while enabling data sharing, the computing device comprising: one or more hardware processors; and one or more machine accessible storage media having embodied therein a policy arbitration system and a plurality of independently executable domains executable by the one or more processors, wherein: the policy arbitration system is executable to manage access to one or more shared system resources by the plurality of independently executable domains; each of the plurality of independently executable domains having domain-specific policies including use purpose contexts that are unknown to the other independently executable domains, wherein the use purpose contexts are indicative of a personal or non-personal use, wherein a personal use context pertains to at least one domain in which there is a higher need to share information than a domain having a non-personal use context, and wherein the non-personal use context pertains to at least one domain in which there is a higher need to protect information than a domain having the personal use context domain; and the policy arbitration system is executable to determine and enforce, autonomously, dynamically in response to an event trigger, a least restrictive combination of the domain-specific policies that enables the plurality of independently executable domains to execute on the computing device for a use purpose context of one of the independently executable domains without violating the domain-specific policies of any other ones of the plurality of independently executable domains, wherein the determination of the least restrictive combination of domain-specific policies includes analyzing candidate policy implementations associated with the plurality of domains and executable to effectuate defined purposes of the domains, wherein the defined purpose specifies the permitted use purpose context of one or more shared system resources of the computing device, and selecting a least restrictive candidate policy implementation that does not conflict with any of the defined purposes of the domains.