My favourite part of this technique is that it should make Drupal much more "deterministic" (?)... You can check your registered callbacks and have a much clearer idea of what will happen on any given function call instead of having to search through every single module and inc file to find something that looks similar to an invoke_all hook call.
-1 on a readability. Sure, it takes a couple of days to pick up the hook concept, but now, I can pop open any text file and know exactly what functions are going to be called when. With a registry, I need to go ask Drupal when they will be called, return to the file, look up the function call I want (which is going to be named mymodules_silly_excuse_for_not_writing_hook_foo), .... I remain skeptical of this plan. I welcome a patch and benchmarks, however. -M