I know next to nothing about this GUID concept, so please correct me if I'm wrong, but isn't this GUID just "yet another unique identifier convention", which may or may not clash, depending on how lucky you are? Would using a GUID really be different than using a hierarchy like Haskell does today?
A GUID is basically a random 128-bit number; it's "just another unique identifer", but it is far less likely to clash than your compiler is likely to be affected by a stray cosmic ray. In the paper I cited, I made the following calculation: Both MD5 (RFC1321, 128-bit) and SHA-1 (RFC3174, 160-bit) are sufficiently cheap, and may be considered random functions for this application~\cite{robshaw96}. Let us consider the likelihood of collisions. For $n$ abstract types and $N$ possible hash values, the probability of a collision is approximately $n^2/2N$. Pessimistically assuming $10^{10}$~programmers in the world, writing $300$~lines of code per day with one abstract type per $100$~loc, the probability of a collision in a century of abstract types (using MD5) would then be $(10^{15})^2/2^{129} \approx 10^{-9}$. This is much less than the probability of a cosmic-ray-induced processor error in this period. (substitute "module" for "abstract type", for the Haskell context; the argument is about hashing module source, but applies to randomly-generated GUIDs equally well, as long as a good random source is used, such as Linux /dev/random, or http://www.fourmilab.ch/hotbits/generate.html). The upshot is that you really don't need to worry about collisions. --KW 8-)