The breakdown goes something like this.
White belts are those who get "a few messages per day." The white belt curriculum has newbs learning how to use stars (not the throwing kind!), use chat to reply to emails, use labels to organize their inbox, clean up the inbox, as well as the proper ways to use search, archiving and themes.
Green belts are those who get "a dozen of so messages per day." They will learn how to use video chat, use tasks, use email filters, highlight important emails, send emails from their phone, preview attachments, use emoticons and the "out of office" messages and plenty more.
Black Belts get "a lot of messages per day." My guess is this is where most of us fall. In the black belt program, Gmail users will learn to use keyboard shortcuts, use search operators, filter different email addresses, use Gears, use the EOM tool, set up canned responses, learn how to use the YouTube and Picasa preview tools and use quick links.
Beyond the black belts are the Gmail Masters. Be frightened of their drunken Gmail fury. These power users will learn how to import Google Calendars and Google Docs into their Gmail, send SMS messages from within email, use Gmail's security tools, change up the RSS feed, and set up Gmail for your own domain.
After completing this course of study, Gmail will not be handing out certificates, trophies or awards, but you'll be able to walk down the street with the confidence that you can kick most other email users' tails.