Business Week writer Arik Hasseldahl has written a brief message on his blog stating that the iPhone SDK may be delayed by 1 to 3 weeks. It appears that the reasons for such a delay are not known but many last minute decisions are being made:
However I’m also hearing that the situation is fluid, and a lot of last-minute decisions are close to being made about what precisely will or will not be disclosed next week, if anything. There are, apparently, a lot of moving parts to something this complex.
Arik originally reported the existence of the SDK a day before Steve Jobs announced Apple's plans to introduce an SDK for the iPhone.