Electronista reports that some developers have already received an advance copy of the iPhone system development kit. Apparently the kit makes good on Jobs' earlier promise of code signing.
The kit is apparently not a full-access type of kit; it's more of an intermediating layer between the programmer and the actual operating system frameworks. That means that a lot of the iPhone's possible functionality won't be realized (at least as the SDK currently stands) as programmers won't be getting raw access to the iPhone. As Jobs said earlier, they want to be as open as possible while ensuring that iPhone owners' information is safe.
If that's true, it also means that a lot of apps that currently work on hacked iPhones via Installer.app probably won't be easy to port over to the official SDK.