My guess is it will be an extension/plugin for Xcode with a possible emulator for testing apps on the desktop. It will be all code, no visual editor. The iPhone is quite limited in terms of processing and memory compared to the desktop, so it won't be like writing VB apps or anything.
I bet they'll allow dashboard widgets for people to make things faster, but allow native Cocoa apps to be written to get better performance and tighter interfaces.