Answering to Jack about the two forms of identity (absolute and in context), my thesis here will be that there is neither any absolute identity of things, nor even maybe anything to identify, but only identification process, upon which both humans and systems have to agree. So, instead of wondering about the nature of identity, maybe we should try and follow an approach similar to the one Quantum Mechanics have introduced in Physics, focus on identification as an experimental protocol, and forget about the "Uncertain Reality" of the subject .
The GAIA exemple posted last week, shows what an identification process looks like in astronomy.
- Collect data following a well-defined protocol.
- Define which configuration of data defines a "punctual light source", otherwise said an object potentially identifiable as a star.
- Define which sets of data (characterized by their types and value distribution) have characteristics conformant to a model of star emission (which is very tricky, since those characteristics vary in a very wide spectrum).
- Compare those data together (each potential star will be observed many times during the mission).
- If possible, compare the mission data to previous data and catalogs to match known objects with the ones defined by the mission (taking into account that a star is a living object, of which characteristics are a priori variable, even on small time scales).
The complexity of such a task, that should yield about one billion objects in the sky, is indeed quite similar to identification of the billions of resources on the Web which can be identified as representing a "punctual subject". Thinking that any subject could be represented simply by a single URI is as naive as thinking that any other star in the galaxy have a simple, single, straightforward and stable "observational signature".
 "Une Incertaine Réalité" is the original title of a book by Bernard d'Espagnat about the status of reality in Modern Physics.
Gauthiers-Villars, 1985. ISBN: 2040164049. http://www.worldcat.org/oclc/420187422
Not sure it's been translated in English, but other of his books have been, e.g :
"Veiled Reality: An Analysis of Present-Day Quantum Mechanical Concepts"
Publisher: Perseus Books - ISBN: 081334087X. http://www.worldcat.org/oclc/30110071