Contrary to popular belief, Microsoft will not be releasing a public Technical Preview at its September 30th event. According to my sources, Microsoft will allow the press first dibs on the new bits (or at least give them a very detailed look), and then release to the public a few days later for testing.
The September 30th event is designed to explain what's coming in Windows for Enterprises, and generally has nothing to do with consumers. The build that will be issued to the public in October will definitely be buggy, Microsoft has attempted to clean up any game-changing bugs, but little ones will still remain present until later in the preview.
There is no embargo for information released on September 30th, journalists invited to the press event will be allowed to live blog everything, and Microsoft will write up their own article detailing the preview and what's new. They'll also talk lots about the Windows Insider Program, something spotted in recent leaked videos and screenshots.
For those who wish to use the Windows Technical Preview, you will be required to join this new Insider Program and sign an agreement saying you allow Microsoft to update your build when it needs to be updated. The Windows Technical Preview is a dynamic test, meaning Microsoft will continue to update it with new builds constantly, adding and removing features, as well as fixing (and creating new) bugs.
The Insider Preview has also been put in-place to iron out users who wish to just play with the builds. Microsoft wants feedback, and the operating system will even prompt you to give feedback after completing certain tasks like sending an email or customizing the Start Menu.
We cannot stress enough that the Technical Preview is pre-release software, it is not designed as production software and therefore should not be used as such. Think about the Insider Program as the Windows Phone Developer Preview program, except updated far more frequently.
It's not currently clear when Microsoft will issue the Technical Preview to the public, right now it's looking like the first week of October, however Microsoft could further delay that date or bring it forward to September 30th if they see fit, right now though the first week of October is looking pretty promising.