Ars Technica has a review of MacOS X 10.4, and, as you might expect, it goes into a staggering amount of detail, from changes in the kernel APIs to the quiet addition of arbitrary file metadata (only a few releases after Jobs consigned resource forks to the dustbin of history and told Mac users to and make do with Windows-style file extensions) and Apple's new hierarchical file-typing system, from internal improvements in Quartz to how Spotlight hooks into the kernel:

Any file i/o that goes through the Tiger kernel will trigger the appropriate metadata importer. This kernel-level integration ensures that the Spotlight indexes are always up to date.

A smart folder could be a normal directory that is specially tagged using an extended attribute (in the "system." namespace, masked-out just like the extended attributes used for ACLs). The actual Spotlight query for the smart folder would also be saved in an extended attribute. The contents of the smart folder would be generated on the fly in response to file i/o system calls (opendir(), readdir(), etc.) and would appear to be a series of read-only hard links to the actual files.

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