We thought based on GoogleGuy's Posts that there would be an update for the Google Webmaster Guidelines. But according to a thread at Search Engine Watch forums named New Google Webmaster Guidelines and Gary Price's SEW Blog entry named Google Adds New Content to "Google Information for Webmasters" FAQ; Explains Supplemental Index, Google has done it early.
As Gary points out, we finally have an official explanation of the supplemental index, in the advanced section.
Supplemental sites are part of Google's auxiliary index. We're able to place fewer restraints on sites that we crawl for this supplemental index than we do on sites that are crawled for our main index. For example, the number of parameters in a URL might exclude a site from being crawled for inclusion in our main index; however, it could still be crawled and added to our supplemental index.
The index in which a site is included is completely automated; there's no way for you to select or change the index in which your site appears. Please be assured that the index in which a site is included does not affect its PageRank.
Plus there is information about the new Google Sitemaps and much more. Good work Google!