We've recently added the telephone area code (NPA) to the zip code information being displayed on the USPS Zip Code Boundary Map page. This new bit of information appears in the infowindow (bubble) when the pushpin marking the zip code is clicked and also appears on the sidebar under the zip code name.
The area code data is current as of July/2007 and is derived from the Maponics NPA Area Code polygon database combined with the USNaviguide's USPS Zip Code Boundary polygon database.