You can inspect your own apps running on device with Reveal without having to resort to jailbreaking. Just link the library with your app, run it on the device, make sure your device is on the same network, fire up Reveal and choose your app from the drop-down app selector.
We've also had reports of people successfully inspecting the Springboard and Apple's apps using Cycript and other means.
You don't even need a jailbroken phone, just a provisioning profile. Objective-C is fully dynamic, so you can override UIView/CALayer so that it sends data to a server whenever it changes and is a child view of the key UIWindow.
They don't say it explicitly, but I would be shocked if Reveal does not work the same when testing on a device.
You would still need a way to get your code running in another process; simply replacing the implementations of methods in your own application would not cause them to change system-wide.
I'd love a tool that could integrate with Jailbroken phones and Cycript to provide run-time introspection on actual devices though.