I have been using the Web Inspector in Safari (Mac) to debug mobile web apps on my iPhone on iOS 6. After upgrading to iOS 7 my iPhone no longer appears in the 'Develop' menu of Safari on my Mac.
I'm currently using an iPhone 4S, OS X 10.8.5, and the latest Safari.
I have checked that 'Web Inspector' in enabled in Advanced Safari settings on the iPhone. I have also tried restarting the device, Safari, and reconnecting the cable.