According to Flash Platform Evangelist Lee Brimlow, Adobe will announce the CS4 suite on September 23rd via a web broadcast that you can register for today. The news is also posted on Adobe.com, so it is safe to say this is official.
Lee is careful to mention, however, that this is not the release date for CS4 -- just the announcement of new features and what one can expect when CS4 is available for purchase. I'm guessing that, like Adobe did with CS3, they will post additional betas such as Flash CS4 to Adobe Labs for everyone to play with after the announcement on the 23rd. Dreamweaver CS4, Fireworks CS4 and Soundbooth CS4 betas are already available for download on Adobe Labs.