Looprumors posts a letter from Apple to the FCC asking for confidentiality regarding the FCC documents for the iPhone until June 15, 2007.
The documents in question include External Photos, Internal Setup, Test Setup Photos and User Manual.
The rumor site speculates that the June 15th date could be the target launch of the iPhone, but, certainly, may simply be an educated guess.
When Apple first announced the iPhone, they claimed a June shipping date. Recent rumors have suggested that this may have been a conservative estimate, and that the iPhone may come as early as April.
Update: Should be noted that the FCC letter has no mention of the "iPhone" product, so it could represent another product, though the timeframe would correlate with the iPhone. Also, it appears the 15th date is likely an estimate rather than a firm date. FCC documents for the Airport Extreme listed a confidentiality date of January 15th, 2007... while the actual release of the Airport Extreme was January 8th.