Grids can form the basis for pervasive computing due to their ability of being open, scalable, and flexible to various changes (from topology changes to unpredicted failures of nodes). However, such environments are prone to failures due to their nature and need a certain level of reliability in order to provide viable and commercially exploitable solutions. This is causing nowadays a significant research activity which is focused on the topic of achieving certain levels of Quality of Service (QoS) in highly unreliable environments (such as mobile and ad hoc Grids). This study will focus on the state-of-the-art analysis of the QoS aspects in Grids and how this is achieved in terms of technological means. A small survey and related work will be also presented. A more detailed analysis on the features of unreliable environments such as mobile Grids will be described. An innovative and efficient mechanism will be described, which is especially designed for such environments, in order to enhance them with the QoS attributes of reliability (fault tolerance through replication of tasks) and service differentiation to the Grid users through a simple task prioritization scheme. The results that this recent research work is presenting are promising for the future advancement of Grid commercialization in such environments.