Is Apple unveiling the iPhone 4.0 OS on Thursday? That's what it looks like! Boy Genius Report has posted an invitation to an event that includes the tagline, "Get a sneak peek into the future of iPhone OS" with the shadow of the number four in the background.
Now that the iPad launch has passed with success, the time seems right to start discussing features in the next version of the iPhone OS. This will presumably run on both future iPhones and the new iPad.
Historically, Apple has released SDK information and details of its next major OS version each March. However, given the release of the iPad and the new information developers needed to prepare software for its launch, it would make sense to wait until after iPad's release to start detailing iPhone 4.0 OS.
If Apple follows its past schedule, the SDK will be available to developers soon and the newest iPhone will be revealed at WWDC this June.