Obviously I'm not going to reveal the fact that I have a gun to a mugger until he's distracted.

I was going to say any mugger would know enough to check for guns before walking off, but then I remembered that crooks aren't an overly smart bunch to begin with...

As for defending in a knife on gun fight, you put your non-shooting arm across your chest, elbow to guts fist to neck. This can help absorb any slashes from getting to your good stuff. (better an arm than a gut I always say). For your shooting arm, you drop it into a "speed rock" position.

All well and good, but that still leaves your femoral artery uncovered, not to mention your face, and half your neck. You simply cannot cover all your vulnerable spots. You can block one, you can block two, but can you block three? My previous instructor demonstrated this one class. He chose one of the guys with faster reaction times (well, faster than me, anyway), took a training knife, and said "I'm going to stab you in the shoulder, then the thigh, then the shoulder. I will not fake. Your goal is to not let any of the cuts hit you." Once the guy said he was ready to defend, the entire demonstration took under three seconds -- the guy blocked the first, was late on the second (but managed to deflect the blow off target), was nowhere close to avoiding the third, and that was knowing exactly where the instructor was aiming. Unless I'm missing something, the only thing you can do to block is move that arm covering your gut hole -- anything else will move your waist enough to throw your speed rock positioned gun out of alignment, potentially causing you to miss -- which puts you at an even bigger disadvantage than the guy in the demonstration.