Apple has sent out a notice to developers advising them that the iTunes App Store has begun accepting iOS 4 binaries for submission and review:
Submit your iOS 4 apps for review so they can be ready for sale when iOS 4 is available to iPhone and iPod touch users. Make sure you have built and tested your applications using iOS 4 and iPhone SDK 4 GM seed, which you can download from the iPhone Dev Center.
Log in to iTunes Connect and upload your iOS 4 applications today.
iOS 4 is scheduled for release on June 21, once again developers aren't likely to get much sleep, especially if trying to once again implement new features for a device they don't have and can't yet test.
(The effort is much appreciated.)