Stock Safari (8.0.2) seems unable to attach to MobileSafari when launched via Tools/Scripts/run-safari --ios-simulator. Nothing shows up underneath "Develop > iOS Simulator > [No Inspectable Applications]". I checked Safari preferences in the simulator, and Web Inspector is definitely enabled.
1. Tools/Scripts/build-webkit --ios-simulator
2. Tools/Scripts/run-safari --ios-simulator
3. Verify in simulator "Web Inspector" setting is enabled through System Preferences app.
3. Open System Safari
4. Try to attach, find nothing.
My guess is this is because webinspectord supports WebContentProcess if it has a private entitlement (which won't be the case in public builds). On OS X we had to fallback to a different approach, we may have to do that here.