Apple's inclusion of the UI Automation component in Instruments with iOS 4 is a definite step in the right direction. It's the first reasonable way to write tests that externally exercise your actual app, rather than weirdly injecting test code into it. It's also the only way to programmatically test lifecycle issues, such as how your app behaves when put in the background, when rotated, when the device locks, etc. Good stuff. Unfortunately, the current implementation of UI Automation also has some significant problems:

1. There's no way to run tests from the command line. The subtitle of the WWDC talk for UI Automation was "find bugs while you sleep;" unfortunately, you can't find bugs while you sleep if you have to wake up to click the "Run" button.
2. There's no way to set up or reset state. The lack of fixtures which set up a known state at the beginning of iPhone tests has been a problem for unit testing (with OCUnit, Cedar, or what have you), particularly for apps that use CoreData. Now it's worse than ever, because UI Automation manipulates the actual state of the app on the device, much like Selenium does in a browser. Sadly, UI Automation provides no method for reseting the device's state, making it nigh impossible to prevent tests from affecting one another.
3. Part of the previous problem is that UI Automation has no concept of discrete tests; it provides no form of organization for your test scripts. No test methods, no set up or tear down methods, just one big stream of consciousness line of execution. Obviously you can break this up into functions as you see fit, but why reinvent the wheel? Since the test script is JavaScript, I like the idea of using Jasmine for this.
4. There's no way to programmatically retrieve the results of the test run. You could debate the value of solving this issue at the moment, considering there's no way to programmatically start the tests either. However, even if you were to write some clever AppleScript to kick off the tests automatically the only indication of the pass/fail status is in the Instruments UI, so you still have to wake up to check the results. I searched around a bit for information on deconstructing the protocol that UI Automation uses to talk to the device, but I came up empty.

I'll definitely use UI Automation, particularly for app lifecycle testing. But, not being able to add those tests to a CI build definitely stings. I very much hope Apple keeps their momentum for automated testing and makes it more developer-friendly.

One of the most common complaints I've read about OCUnit, the unit testing framework built into Xcode, is that the tests you write with it won't run on the device. In addition, I personally have found the process of setting up a target for tests that depend on UIKit confusing and onerous. So, one of our goals for Cedar was to make testing UI elements easy (or easier), by making it easy to run specs in the simulator or on the device.

Probably the second most common complaint I've read about OCUnit is that the tests run as part of the build. This makes the test output difficult to separate from the build output, and makes it impossible to use the debugger when running tests. So, in addition to making it easy to run specs on the device, we wanted to be able to run them as a separate, debuggable executable.

Finally, we consider it important that our specs run in our CI system. That means we wanted to be able to run Cedar specs from the command line, and get an exit code signifying success or failure. At the same time, some of us appreciate the value of the green/red feedback for specs passing and failing, so sometimes we like a nice UI. As of today, Cedar will accommodate all of these various requirements.