Confirming that the iPhone will launch at 6pm on June 29th, Apple have given opportunity-hungry developers some news about how they'll be able to code for the covetable handset. Based on the full Safari engine in iPhone you'll be able to write Web 2.0 applications that work just like hard-coded apps on the phone, integrating with cellular services and fully secure, without an SDK necessary.
Being html and AJAX based means instant distribution is possible; all you need are web standards. Testing can be done in Safari on your desktop, then simply transferred to the iPhone. As an example, Jobs demonstrated several applications running in Safari on an iPhone: a custom corporate address book database which used ldap and took less than one person-month of coding time (and in fact just 600 lines of code), contacts from which could be dialled direct as if from the iPhone contacts. Both the Apple and the custom applications looked identical, and each were able to access other iPhone functionality such as Google Maps.
Expect to see big names such as Salesforce and Google using this system to provide iPhone software shortly after launch!