John Gruber has posted More Questions on his Daring Fireball blog about the iPhone SDK. I can answer one of his questions.

• So it seems like the answer to my question yesterday about how users will be prevented from running apps downloaded directly from developers (rather than through the App Store) is that unsigned apps will only work on your iPhone if you pay (and get approved) for a $99 iPhone developer account. But does that mean that approved developers will be able to freely exchange unsigned apps with each other?

An unsigned app will not run on an iPhone at all.¹ The only place an unsigned iPhone OS application will run is on the simulator.

I doubt that the barrier for the $99 iPhone developer program is higher than for other types of code signing certificates. The iPhone code signing certs are issued to either individuals or organizations. Apple’s approval for the App Store will be on a application by application basis.

If you as a developer get an application or two approved for the App Store and it later turns that you’re breaking Apple’s rules, it seems safe to presume that one of Apple’s possible recourses is to revoke your certificate and all your (as in signed with your cert) iPhone OS applications will stop working.

¹ iPhone OS Programming Guide: Working with a Device