This is surprising, as I was told last year (2005) by my postman that no mail would be forwarded overseas. Here's a clarification from the USPS.gov website:
>>International Mail
* Express Mail®, Periodicals, Package Services, and Standard Mail™ items are not forwarded Internationally. In addition, First-Class and Priority Mail® items that contains a non-forwarding endorsement or appears to contain merchandise are not forwarded Internationally.
* First-Class Mail® and Priority Mail® cards and unregistered letters are forwarded Internationally, for 12 months, as long as both of the following conditions are met:
1. The mailpiece does not contain a non-forwarding endorsement
2. The mailpiece does not appear to contain merchandise.
* The post office will advise publishers of a customer’s new address.
* All international mail that is not forwarded will be treated as RTS (returned to sender) or waste, depending on the class of mail.
* Note: For items originating outside of the United Stated that are sent to an addressee who has moved within the United States, refer to Forwarding of Inbound International Mail.
More on mail forwarding:
If you are moving to an international location, or have a Military (APO/FPO) address, you fill out the same Change of Address (COA) card that is filed for a domestic move (PS Form 3575).
* This change of address process must be completed at the local Post Office™ (i.e. the U.S. office).
* If you are moving to or from an International or Military address the COA can not be submitted via the internet.
If you forget to submit a COA and you move to the international or Military address, you can write a letter to the old and new Post Office facilities explaining the situation.
* Provide direction on how / where you need mail forwarded.
* The letter should include as many details asked on the COA card as possible.