As you can see from the graphic, the date is 9/12 and the '12' is casting the shadow of a '5' indicating the 'iPhone 5' that has been rumored for months now.
AppleInsider.com, who received an invitation, writes:
"The invitations sent out read "It's almost here," with a large number 12. Below the date of the event is a shadow cast with a number "5," implying the unveiling of Apple's next-generation iPhone, the successor to the iPhone 4S.[Via AppleInsider.com...]
The event will be held next Wednesday at 10 a.m. Pacific, 1 p.m. Eastern at the Yerba Buena Center for the Arts in San Francisco, Calif."