Thanks to Jason Ralph for his clarification and, let me note, that I did assume that his first post was positioned to discuss the historical development of the laws of war rather than holding them as normative.  This is one of the interesting, and fun, things when we talk across disciplines.  I'd like to expand on the idea of the laws of war as being culturally contingent, and expand that observation beyond the discussion of the Peace of Westphalia.

First of all, I think it is useful to introduce a concept from Anthropology to the discussion: a "culture area".  A culture area is a geographic region that contains a number of different societies. They are organized along roughly similar cultural institutions, have roughly similar technology, and share a similar economic base. The concept appears, on the surface, to be similar to that of a "Civilization", but it is actually quite different and may well include multiple "Civilizations".  An historical example of this would be the five Empires period in the ancient Near East (ca. 1400 bce) which formed a "culture area" despite including such diverse civilizations as Mari, Babylon, the Hittite Empire, Egypt and the Minoan "Empire".  Another example of a culture area would be Europe in the 17th century.

I would suggest that what might be called "conventions of competition" appear within each culture area as a way to regulate the destructive effects posed by competition.  Now, I'm using the term "competition" rather than "war" for the simple reason that I would ague that "war" is a sub-set of competition - extrapolating from Clausewitz, war is an extension of politics, politics is an extension of competition, and both are culturally constructed perceptions held by populations that are subject to change over time and in response to intrusions from outside of the culture area (as a note, Keith Gomes' article "An Intellectual Genealogy of the Just War" tracks these changes in one such "convention").

Jason's point - "if we apply IHL to those fighting on behalf of these kinds entities [trans-national non-state actors] (either to prosecute them or to protect them) then we are helping to change the character of war as a social institution" - is quite correct, although I would have phrased it as "both as a social perception and as a social institution".  I would suggest that, at the perceptual level, understandings of "war" have already shifted in part as a result of the increasing looseness of the term's application (e.g. War on Crime, War on Drugs, War on Poverty, War on Terror, etc.). 

I would also suggest that there has also been a de facto acceptance of trans-national non-state actors as "state-like", as shown by the PLO, the ANC and other "national liberation" groups.  I characterize them as "trans-national" because, while they were focused on achieving statehood in a given territory, their actions were not limited to those territories.  I would also note that this is exactly what Al Qeada is doing - it is focused on achieving "statehood" over a given territory (the old Caliphate and whatever else it can grab) while operating outside that territory.

If the perceptions are shifting, is it not also appropriate to shift the formalization of those perceptions?