Apple has released the developer build of iOS 5, which features a brand-new notification system, Twitter integration, and much more. If you can’t wait until the Fall to install the next major release of iOS, or need to debug your next application, hit the link below.
Developers only, please! Also be aware that downloading and installing will mean that you can’t downgrade to iOS 4.x. Ever, according to Apple; so exercise a bit of caution.