Digital cameras win hands down. Slam the power button, slam the photo button, and you have at least a blurry blackmail-worthy shot of your friend dancing with a hobo.
With a mobile phone, by the time I get the app up and running, the hobo is gone and my friend is left with nothing but hobo sweat.
This is even quicker with the "NoLock" app that disables the lock screen.
Even on the relatively poor shutter speed iPhone 4, I'm usually taking pictures within 2-3 seconds. Supposedly the 4S has significantly reduced the shutter-lag to first picture - looking forward to seeing the truth of that.
How is that sequence so significantly different from (in iOS 5) double pressing the home button from the lock screen, touching 'Camera' then taking a photo? I'm not suggesting that camera phones are as good as compact digital cameras yet, but it's getting much closer.