Apple announced that in the first 4 days of the availability of the iPhone SDK 100,000 developers downloaded a copy.

Apparently AOL, Electronic Arts, Epocrates, salesforce.com and Sega have already demonstrated "amazing" applications using the SDK.
Despite limitations like single 3rd party application execution, SDK limitation for communication programs, and iTunes application distribution with revenue share for commercial applications, the iPhone as a software development platform is on its way.
Apple can always allow more and open up more. Like with most new things you cannot easily take things away once you have allowed them. So the stringent strategy of Apple is understandable as long as the growth and adoption is high enough.

Sure a lot of the 100,000 SDK downloads will not convert to registered iPhone application developers, but it shows how strong the interest is in the developer community to develop for the sexy touch screen phone.

More details on the iPhone SDK Site.