Let me get the disclaimer out of the way up front. I'm a cook and food writer - not a microbiologist. The information presented here is a result of my research into the subject. And I am not making any recommendations. But I've found this information helpful in determining what's safe when it comes to cooking meat.
We're taught that food should never be held at temperatures between 40 and 140 degrees Fahrenheit. That's because most bacteria will quite happily reproduce in that range, but reproduce very slowly, if at all, below 40 and above 140. But note, the temperatures at which bacteria is killed varies according to the microbe. For example, Salmonella is killed by heating it to 131 degrees for one hour, 140 degrees for 1/2 hour, or heating it to 167 degrees for 10 minutes. You can see from this, then, that when it comes to killing microorganisms both heat level and time affect the equation.
There's also the issue of where the contamination is found. E-coli lives in the intestinal tract of animals - not the flesh. And the danger is that in the process of butchering a cow or chicken some of the contents of the intestinal tract may contaminate the exposed flesh. That's why it's safe (relatively) to sear a steak over high heat and still eat it rare or medium rare (125 - 135 F). That's also why all ground meat should be cooked to 160 degrees - because the external flesh and internal flesh are mixed together during grinding.
Trichinosis, which is a multi-celled parasite and not a bacterium, lives in the muscles and so searing the outside of, say, a pork chop won't kill any organisms in the meat (though it will taste better). Fortunately trichinosis is killed at 135 degrees so it's safe to eat if cooked to at least 140 or 145. Salmonella can also sometimes inhabit the meat of poultry so cooking chicken and turkey to at least 160 degrees is wise. (Note that salmonella can also inhabit eggs and so there is a risk with soft-boiled eggs, omelettes, and scrambled eggs.)
My take-away from all this is that it's safe to cook meat (and vegetables) at low temperatures for longer periods or higher temperatures for shorter periods. And it's almost always safer to sear meat over high heat before cooking it at lower temps.
But heat alone isn't the only factor in preventing food poisoning. There's also a toxicity factor. Some bacterium are simply more toxic than others and some toxins hang around after the bacteria is dead. Most of us with healthy immune systems can ingest a bit of salmonella or listeria and our systems will kill it off without our even noticing. Botulism toxins, though, are highly potent and dangerous and even a small dose of the bacteria can have significant effects. Botulism occurs mostly in improperly canned goods but can also appear in homemade sausage. Never take a chance on something that might contain botulism.
I do a lot of low-temperature cooking both for roasts and braises, but I usually brown the meat first over medium-high heat - about 350 degrees. I serve pork cooked to medium (145 degrees) with complete confidence but I feel safer grinding my own meat. (In a commercial establishment, one piece of contaminated meat can potentially spread that contamination through several hundred or thousand pounds of ground meat, which is why there are outbreaks of food poisoning.)
If you want to be exceptionally safe, follow the USDA and FDA guidelines and cook everything to at least 160 degrees Fahrenheit. Also, never hold hot food below 140 degrees and cool food you're going to refrigerate quickly to at least 40 degrees. Me? I'll keep eating rare steaks, medium pork, and scrambled eggs.
You might also want to read this tip on kitchen thermometers.
Note: This tip was prompted by a question Linda Larson, BusyCooks.About.com, had about the new hotter (read faster) slow cookers. Although I have a 20-year-old slow cooker it's only been used once. I prefer my somewhat abused Le Crueset cocotte (enameled, cast-iron Dutch oven). But I know many excellent cooks who swear by their Crockpots and I freely admit I'm a curmudgeon. So you might want to check out Linda's take on the topic. And while you're at it, Stephanie Gallagher at Cooking for Kids has a good piece on lettuce safety.