I just learned about this and thought it's worth to share. 

Currently, the U.S. Postal Service (USPS) is using the Postnet Code and PLANET Code to track mailpieces. The Postnet Code is a special barcode that encodes ZIP Code info, which it improves the speed, accuracy and delivery of mail. The PLANET Code tracks inbound and outbound mail and provides confirmation of mail delivery.

By May 2009, USPS is going to launch the new mail monitoring system called Intelligent Mail, which improves processing efficiency and service performance by combining the capabilities of both the Postnet and PLANET codes. The Intelligent Mail Barcode is a standardized thirty-one digit barcode located on a single mailpiece that allows businesses to view their mail at every step of the mailing process.

This new system has many benefits for commercial mailers including the following:

• Mailers have an advanced mail tracking system
• Mailers can check for address quality automatically instead of manually
• Mailers can identify improper pre-sort mail handling automatically
• Business mailers have additional space on envelopes which can be used for marketing messages
• Mailers have real time feedback on mail quality
• Mailers have the ability to identify when customers receive products, bills, or advertisements in a more timely fashion