via Decapitator this morning
We should start seeing posters, messages on the website and GW staff telling us to be in stores on the 17th September for a 'Special Announcement' soon. Could be wrong but I'm fairly certain that this involves the 'Dread Fleet' the fantasy naval game special release. It's also Parade Day for 'Armies on Parade' so it's another way of getting people in store, people will be advised to register on the website so that they don't miss out.
Also White Dwarf has been pushed back from the 24th September to the 1st October, not sure if this is Dread Fleet orientated or just that September is a long month.
Please take note that the push back for the White Dwarf release date is probably due to Games Day UK being on September 25th, and they would like to make any upcoming big news about releases (like Necrons) there instead of us getting wind of it through the White Dwarf before hand.
Here is a little more via Bramgaunt Regarding the 17th
There is no Event of any kind. Stores will not recieve a preview copy, only promotional posters, which we already saw, the Wanted posters. Preorders will go up on the 17th. Stores are supposed to invite as many people to promote the game and create a hype, but you don't miss out if you spend the day at home and click refresh on the browser once or twich the hour. They expect the site to crash, btw (due to hundreds of preorders).