The SDK was around from the beginning, but planned for a curated channel of third parties that Apple selected itself. Apple wsa incredibly worried about security issues on the device, and initially tried to satisfy everyday developers with: http://www.apple.com/pr/library/2007/06/11iPhone-to-Support-...

The outcry was so fierce and sustained that around 3 months later, Apple backpedaled on this strategy and Jobs himself announced that they would open the platform up.