Email is centralized yes but its also somewhat open. You can send an email to any provider and receive from any provider. You cannot send a message from say facebook to twitter and get a response back.
*Google Wave enters, stage left.*The issue with email is that innovations have to be adopted independently everywhere for them to be widely useful. But that doesn't have to be the case. You can structure a decentralized system so that any user of the system can deploy innovations on it, that the other users can immediately benefit from, without requiring them to be standardized.