iOS 5 won't be out until October 12, but if you're a registered iOS developer, you can get your hands on the gold master release now.
The Gold Master update is available for download from Apple's developer site, but unlike previous builds of iOS 5, which have offered updating over-the-air via iOS' Software Update, the final beta is strictly a download-only affair. If you try to install the update directly from your iDevice, you'll be told that the software is up-to-date, even though it isn't..
Developers who log into the iOS Dev Center will first need to accept an updated iOS Developer Program License Agreement before they can get their GM download started.
As MacRumors notes, this final beta build joins the XCode 4.2 GM, which is also now available from Apple's developer site. In order to use the GM, you'll also need iTunes 10.5 Beta 7.
iOS 5 introduces an all new notifications system, a free messaging service by the way of iMessage, deep Twitter integration, location-aware reminders, various updates to the camera app, and more.