Based on prior history, the ship date will likely be shortly after the announcement. Popular rumors put that at October 14th, but nothing official has been stated yet.

iPhone, announced Jan 7, 2007. Released in the US June 29th.
iPhone 3G, announced June 9th, 2008. Released in the US July 11, 2008.
iPhone 3GS announced June 8th, 2009. Released in the US June 19th, 2009.
iPhone 4 announced June 7th, 2010. Released in the US June 24th, 2010.
iPhone ? announced October 4th, 2011. Released in the US ?