The documentation can be accessed by pressing F1 from inside the Lightroom 3 Beta application, or using a web browser on Adobe's support website. The help docs are split into 15 main sections, guiding the user through their workflow from import through viewing, managing, organizing, developing, exporting, editing in an external application, creating and viewing of slide shows, printing, and creating web galleries.
There's also a What's New section that helps users familiarize themeslves with the beta, and will likely serve as a good starting point for those who've just downloaded the beta.
As listed in the Phosphors article, some topics of particular interest related to the changes and new features in Lightroom 3 beta are as follows:
For more details, read the entry on the Phosphors blog, or just browse the Lightroom 3 Beta Help site.