Rumor has it September 12th for Target. I read this at Kindle Boards, someone who is a friend of someone who works at Target had been told this. It is the only date I have heard but it is essentially a rumor heard through a second cousin.
Staples said that they would get them in the Fall with the DX coming for the Holidays. And now Best Buy will be selling the Kindle but no release date was listed in the announcement.