Many feel that Google's invite-only distribution of Gmail accounts is a shrewd marketing move designed to create artificial demand. More likely, they're just rolling the service out slowly; it is still in beta testing after all and 40 million people at once would probably have been a nightmare for them to deal with.
But wouldn't it be fun if the real reason that Google is distributing accounts the way they are is to build whitelisting into their system? With the Gmail economy that's sprung up to facilitate the trafficking of invites (now somewhat curtailed), not all Gmail users are known by the people who invited them, but certainly some fuzzy whitelisting could be utilized to improve Gmail's spam filtering.