You mean, for example, using Safari to debug mobile Safari / webview-based apps? That'll get you about 80% there, but environment differences (i.e., memory and processor limitations) on the device and quirky rendering behavior in mobile webkit almost always result in weird bugs that traditionally have required a lot of poking around blindly to fix. Differences between the simulator and the actual device are fairly common too.

It's only fairly recently that useful debugging tools like this and the built-in iOS 5 stuff have emerged.