"Gun or not, Martial Arts or not, you're fucked in this scenario. If I am so completely fricken unaware that some dipwad could get that close to me without my permission, his ninja-ass deserves my wallet. "
Your second sentence is right on point! but I disagree that you're fucked. There are ways to disarm an attacker when he's behind you - - not saying they're easy and I'm certainly not saying they're dangerous, but if you're in that situation and have the choice between maybe dying and absolutely dying, I'll take the maybe alternative
re: the slide:
No, I never said that jamming your hand in a slide will fail the weapon permanently. I said it will stop the weapon from firing. And it will.
Of course, it'll also pinch the holy hell outa your hand (esp. if it's a hammer rather than a slide), so when I did this disarm a couple of years ago in a mugging I wound up with a big infected wound in the web between my thumb and finger for a week, but I'd say it was worth it