At Photokina this year, the Metadata Working Group standard was announced. This is an industry wide effort to improve how metadata is written and read for a number of key fields used in consumer workflows. The formation of this group included members from Adobe, Apple, Canon, Microsoft, Nokia, and Sony. Most recently, both the IPTC and PLUS organizations endorsed the efforts from the Metadata Working Group.
The specification produced by the Metadata Working Group is available from the website. It outlines the goals as well as details how each of the fields should be used in order to ensure maximum compatibility among applications that adhere to this specification.
For more details on the Metadata Working Group, check out the website.