iPhone OS 4.0 SDK iOS 4 SDK is almost (but not quite) ready to roll for all members of Apple's developer program, not just the paid App Store developers. However it's likely to be a week or two before everyone gets access to the GM release. Right now, it's still going to be a release candidate.
Paid devs are almost ready to finally start shipping apps leveraging the new firmware's amazingly cool features. We can't wait until we start seeing VoiP and Game Center apps in action.
Today, you'll be able to head on over to Apple's developer site and start downloading your copy of the 4.0 SDK release candidate. Although the site still shows beta 4, the gold master release candidate, as Steve announced at the Keynote this morning, should be ready momentarily.
Of course, the iPad SDK is another matter. Any iPad SDK, when it appears (likely in the fall sometime), will remain under an ongoing NDA until that firmware and SDK is officially released. But the good news is that the new version of the iPhone firmware is just around the corner on June 21st.
UPDATE: The developer site is DOWN.