Code Signing and You
Code signing itself is a neutral technology, but it gives incredible power to the system vendor, and that power is just waiting to be exercised and abused. I believe that the iPhone is serving as a testbed to see how users and developers will react to an environment with ubiquitous code signing and control. If it goes well I think we can expect to see our desktop Macs gradually move in this direction as well.
The purpose of signature for Desktop is not exactly the same as for an iPhone.
On Mac, it's a powerfull toll that can be configured by the user/administrator to restrict code execution, but it MUST remain under the machine administrator 's control.
An network admin must be able to says: I want that only Apple's code, and applications approved by me run on those machines.
And so, I don't think Apple plan to restrics the execution of applications on desktop, but they will give the admin a way to do it.
On iPhone, it look like they plan to use the "Only code signed by Apple" restriction.