John Gruber of Daring Fireball takes a closer look at Spotlight and Dominic Giampaolo’s modifications to HFS+ which enable the rich desktop search.
Some interesting issues.
[T]he new Tiger version of Mail stores each message as a separate file that allows Spotlight to effectively return individual mail messages as search results. No other major mail client uses a one-message-per-file storage format.
Importers are fired on every file the moment it is created, saved, changed, or moved, including when files are made available through a newly mounted drive.