[UPDATE: Engadget has posted that the document referenced is, in fact, related to the new Apple Airport Extreme (FCC BCGA1143) not the iPhone.]
In a not-so-surprising move, Apple filed for confidentiality with the FCC on October 12, 2006 for almost everything relating to iPhone. The document obtained by LoopRumors (picture courtesy of WorldOfApple) details Apple's request:
...we are requesting the Commission withhold the following attachments as confidential documents from public disclosure indefinitely.
Theory of Operation
The above mentioned documents contain detailed system and equipment descriptions and are considered proprietary information. The public disclosure of the above mentioned documents might be harmful to [Apple] and would give a competitor an unfair advantage in the market.
The manufacturer may request Confidentiality of the information from the FCC. This usually involves an additional $155 filing fee. If Confidentiality is not requested, all documentation will appear on the FCC website at the time the certification is granted. With the optional fee, the manufacturer can typically eliminate all design documents from this posting process.
What's interesting about the leaked letter is near the end:
...we are requesting the commission grant short-term confidentiality to the following attachments until June 15, 2007:
Test Setup Photos
June 15th is the final day of Apple's Worldwide Developer Conference (WWDC).