Today Apple released the iPhone SDK in a beta format for developers. The official version and the ability for the average joe to use these to be developed applications will come out in July. However it appears that Apple greatly underestimated how many people want to produce applications for their now infamous device. The link to get the SDK and subsequent information is http://developer.apple.com/iphone/program/, but unfortunately this site is suffering from a massive digg effect as it has not be available since the announcement was made almost 4 hours ago.
UPDATE:
It looks like Apple is struggling with keeping their servers up. I was able to get to the SDK page a little while ago, but a lot of the links were broken and I didn’t try downloading it (I am away from my Mac at the moment). Sporadic attempts to get through to the proper page may occur.
You also need to have an Intel Mac in order to proceed, so if you are like me and waiting to buy a new Mac, now is the time.