Developers signing on to the iPhone SDK program are now expressly forbidden from writing iPhone apps that can be installed via jailbreak, or any software for any Apple technology that messes with security or DRM.
Here's a piece of what Ars is quoting from the new agreement:
You will not, through use of the Apple Software, services or otherwise, create any Application or other program that would disable, hack or otherwise interfere with the Security Solution, or any security, digital signing, digital rights management, verification or authentication mechanisms implemented in or by the iPhone operating system software, iPod touch operating system software, this Apple Software, any services or other Apple software or technology, or enable others to do so...
It also says that anyone using the SDK to develop software can then only get distribution through the App Store, and at Apple's sole discretion. As Ars points out, it's not going to dry up jailbreakers, but it will make those gray area developers a little more skeered of swerving from the path. [Ars Technica]