How can I implement Schema.org on my Drupal 6 site?
Anyone coming across this looking for this in Drupal 7 (I know the question is for D6), see the Schema.org module. Using this module sites administrators can specify what schema.org terms they want to associate with their content types (e.g. Art Gallery, House, Event, Person, etc.) and their fields (description, participants, etc).

Although this project is syntax agnostic, the schema.org types and properties selected by the site administrator will be used as part of the native Drupal 7 RDFa markup.

You can also use the Microdata module. Microdata is a module which takes care of embedding schema.org terms in HTML per the microdata spec. Core does it via the RDFa 1.0 spec. There is a UI for schema.org in microdata so that it does not have to depend on other modules for the UI (Drupal core does not include such UI).