The link below points to a paper that I wrote a few years ago regarding Immutability in Software Configuration Management.
Abstract. A basic function of Configuration Management (CM) is to accurately reproduce past and present information. Many other aspects of CM depend on this function. Flaws in this function affect and are magnified by other aspects of the system. Ideally, a CM system should perform this basic function automatically and flawlessly without any special knowledge or effort on the part of the users. This paper introduces the Timesafe ® property to describe CM models that possess the capability of accurately and automatically preserving and reproducing past and present information. As a first step towards enabling the proof or disproof that a CM model is Timesafe, this paper provides a formal description of the Timesafe property and the currently known list of counter-examples.
The paper can be found here:
http://www.accurev.com/blog/Timesafe%20-%20Immutability%20in%20CM.pdf
This is one of the core competencies that I believe to be integral in Software Configuration Management products, especially in the dynamic, distributed and parallel development environments that we as CM professionals must try to manage.