On my Galaxy II, whenever I want to have the camera functionality ready, I open the camera app and press the power button. The screen turns off, and I simply need to wake the phone back on to be immediately ready to take a blackmail picture.
This is even quicker with the "NoLock" app that disables the lock screen.
My Finger DNA has already memorized "Double Click + Camera Icon" from the lock screen, and then hit the shutter release/+ button on the iPhone like mad.
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.
This is even quicker with the "NoLock" app that disables the lock screen.