Patently Apple has turned up a curious letter. It was sent on June 4, three days before Steve Jobs unveiled the iPhone 4, to the Federal Communications Commission — the government agency that must certify any wireless device before it can be marketed in the U.S.

Signed by Apple’s (AAPL) wireless compliance manager, it requests confidentiality for a wide range of documents the company had submitted with its application for certification — from exterior photos and the user manual to block diagrams, schematics and bills of materials.

“Although Apple has begun to market the device publicly,” the letter states, “these documents reveal technical and design information that has not been publicly disclosed.”

Fair enough. What’s curious about the letter, however is that it requests short-term confidentiality for images that had already been revealed by Gizmodo and permanent confidentiality for details that the company just made publicly available on iTunes. On Thursday, Apple posted podcasts of every one of the 126 technical sessions it hosted last week at its Worldwide Developers Conference — sessions packed full of specs and other details that the developers who attended had to promise not to reveal.

Although you must be a registered Apple developer to download them, registration — like the podcasts — is free.

Below the fold: Apple’s letter to the FCC, courtesy of Patently Apple.

[Follow Philip Elmer-DeWitt on Twitter @philiped]