defaults write -GlobalDomain WebKitDeveloperExtras -bool true
...and then launch (or, quit and relaunch) any app that exposes a WebKitUIView. Right-click on the WebKitUIView and choose "Inspect Element" - Bingo! Up pops the WebKit web debugger.
Now, some applications block this ability unintentionally, or sometimes very intentionally (Mail.app, I'm looking at *you*).