I've had remarkable luck in rarely or being only a little sick in my "developing" world street food experiences. The advice BR gives is quite useful. I'll add a few things:
1) Sometimes, whether because of sheer laziness or a powerful urge to try something, you may consume something that you immediately recognize will probably get you sick. Forget antibiotics, charcoal pills and other hoo-dads. All you need - I am serious - is two shots of hard liquor. Two shots, maybe three. Hell, have four. Of course, obtaining the hard liquor will be difficult in certain Muslim countries, but you will be fine in Southeast Asia.
This technique has saved me numerous times. From street oysters in Phnom Penh (is that the ocean or a murky river over yonder??), to undercooked street meet in Cairo, this technique has steeled me from the worst ravages of G.I. hell. It is absolutely crucial that you get the hard liquor in your belly within 30 minutes of eating the poisoned food in order for this to work.
2) Don't be afraid of street food. Eat it as soon as you arrive at your destination. If you do get sick, it will be within the first few days and you will have uncompromised immunity thereafter. Do this a few times, and you should be able to travel the world without getting sick.
3) Don't take your pretty, hyper-hygenic girlfriend to street food stalls in Cairo or Mumbai. She will get really sick. And you will laugh. And she will be mad at you, and never trust you, and you will need to buy her stuff.
4) If you do get sick and die, you will be dead, and death is like a long sleep with no pesky alarm to wake you up. So who cares!
But seriously (and I am being VERY serious), I ate at many markets and stalls like the one David photgraphed in Sihanoukville while I lived in Cambodia and never experienced more than a single, minor bout of the runz. I didn't shy away from unpeeled fruit, fresh veggies, juices, etc. (when did those become the devil?).
And on a final note, perhaps the most frightening experience I have had eating abroad was at a tiny, dingy truckstop bathroom of a restaurant in Cairo. After ordering a salad and some fried chickpea balls, I glanced over at the vegetable cutting station and noticed two workers frantically trying to plug a drain that was overflowing, nay shooting two feet in the air, with brown, bubbly water. Needless to say, I ran the hell out of there without even saying a word to the proprietors, and never looked back!
"By the fig, the olive..." Surat Al-Teen, Mecca 95:1"