Adobe has taken the unusual (for them) step of "preannouncing" the announcement of CS4. In other words, they're telling the public that they will announce (not ship) CS4 on September 23. You can sign up to view a Web broadcast of the announcement here.
While this may seem awfully soon on the heels of CS3, its right on schedule with the timeline that Adobe has discussed publicly of releasing a new Suite every 18-24 months. CS3 was released in March of 2007.