From the Rogue Amoeba Blog post Code Signing and You:

However, the environment is different in one important way. Apple is the gatekeeper:

“Phones will only run apps signed by Apple. It also applies FairPlay to the package.”
Twitter message from Deric Horn on March 6, 2008

Let me repeat that: if Apple doesn’t sign your iPhone app, it does not run.

This makes more sense than what I was thinking. It means an iPhone OS device doesn’t need to know thousands of valid certificates.

In addition to upgrading applications will the App Store be capable of removing applications?