And...it's LIVE! The 4.0 SDK for the iPhone OS is now available to developers. Head on over to Apple's iPhone Developer site (login credentials needed) to download the SDK disk image and get started.
As with previous beta releases, the new firmware and SDK remains behind an NDA. This NDA will likely remain in place until the 4.0 goes live, which is expected to happen sometime in late spring.
The new SDK will support features announced earlier today at the iPhone OS 4.0 roadmap media event.