The planning of the event was revealed by the Apple rumour site AppleInsider, which notes that although Apple hasn’t officially announced what’s happening, it also hasn’t protected the secrecy of the event with the iron fist usually reserved for its biggest events. So we probably shouldn’t expect the iPhone 5 to appear.
One of the most plausible reasons that have been proposed is that retail employees will be introduced to John Browett, the new head of retail at Apple and the successor to Ron Johnson. That idea was put forward by Gary Allen of ifoAppleStore.
Further details will arise closer to the date.