Beta 4 of the iPhone SDK has been released:

The fourth beta adds support for Open GL ES within the iPhone Simulator, rendering 3D graphics in the Simulator that mirror behavior on the iPhone and iPod touch. There are new iPhone OS templates in the New Project and New Target assistants. Code signing is now enforced on devices.

The Known Issues section elaborates on code signing:

Code signing is enforced on the device starting with beta 4 of the iPhone SDK and iPhone OS. This means that your project will need to contain a valid certificate signing identity that resolves to a valid certificate in your keychain. If there is no valid identity in the project, the code will continue to function on the Simulator, but will fail to execute on any device.

I’m confused. Didn’t the device always enforce code signing, and fail to execute code built with the previous betas? I couldn’t even get my iPod Touch to load any code, although it’s well possible that’s because I don’t know how loading the Touch is supposed to work. I’m going to keep my eyes open for a better explanation of beta 4’s code signing.