For starters, there's the wording: "Please join us to learn about the iPhone software roadmap, including the iPhone SDK." Why use the word "roadmap" at all? Why not simply say: "Please join us to learn about the iPhone SDK" or "Please join us for the unveiling of the iPhone SDK"?
The word "roadmap" suggests a journey. And a journey takes some time. It makes me think that we are going to learn new details about Apple's plans for the SDK next week, rather than actually see the release of the SDK.
Then there's the graphic image of the roadmap itself, which shows a road with markers for "Enterprise" "SDK" and "Software Update" spaced out along the way (in that order). One interpretation of this is that new Enterprise features will be made available (or even just announced) first, followed by the release of the SDK to developers (perhaps a beta version initially), followed by an iPhone software update that (finally!) allows users to obtain the SDK-dependent software.
Within such a framework, the SDK could be out within a few weeks while the Software Update may not become available for another several months.
And yes, this is all just speculation. Perhaps I am over-interpreting the tea-leaves here. A week from now, we'll know for certain.