If it remains unclear why the Apple Stores are closed already for 2 hours, one can speculate that Apple needs to prepare everything to get the AppStore up and running in couple of hours, as well as releasing the Firmware v2 and having ready the new Mobile.Me service. Some rumors indicate that Apple could launch the iPhone Firmware v2 as well as the AppStore today.
Altogether quite some work, without forgetting that it might also include some new products or revision of existing models. One can imagine that several iPhone 3G-dedicated peripherals will have to be entered in the Apple Store.