I put some uncooked stew meat and veggies on this morning and left for 5 hours. When I came home I realized it was only set on warm. Is my meat safe to eat? I have now put it on the stove to cook.
jdev is right about the USDA's cooking temps, but I would be very surprised if a warmer setting got your food above 140 degrees F (60 C). When food is under that temperature, bacteria reproduce almost geometrically. Later cooking will reduce your risk but not eliminate it. In your case, if you didn't spend quality time in your bathroom last night, you dodged a bullet.
Most of the bugs that make us sick don't grow well below 40 degrees F and are killed when heated above 140 degrees F. In particular, E coli, Salmonella (various types), Vibrio (various types), and Listeria monocytogenes, which are the most common of the bacteria that give us food poisoning, are all killed by heat. We tend to get sick from them mainly because we ate raw/undercooked food or the bugs got into the food after the cooking is done.
There are a few nasties, however, that can leave behind heat-stable toxins even after they have been killed -- most notably, Staphylococcus aureus. Staph a is a very common bacteria, the source of many illnesses. If your food was contaminated with Staph a before you cooked it, and they had time to multiply, then even though the bugs themselves would have been killed by the cooking, their toxins would still be in the food -- and you would have gotten to know your toilet very, very well (symptoms at both ends of the GI tract begin within about 1-6 hours after eating and are severe, but they resolve in 48 hours).
As a general rule, don't eat or use meat, dairy products, or cut fruits/vegetables that have been in the "danger zone" (40-140F) for over 4 hours, even if you planned to cook it later. (Obviously, there are a lot of exceptions to this rule, but that's another long post.)
It all depends on the temperature of your warmer.
The USDA recommends the following minimum internal temperatures:
Cook all raw beef, pork, lamb and veal steaks, chops, and roasts to a minimum internal temperature of 145 °F as measured with a food thermometer before removing meat from the heat source. For safety and quality, allow meat to rest for at least three minutes before carving or consuming. For reasons of personal preference, consumers may choose to cook meat to higher temperatures.
Cook all raw ground beef, pork, lamb, and veal to an internal temperature of 160 °F as measured with a food thermometer.
Cook all poultry to a safe minimum internal temperature of 165 °F as measured with a food thermometer.
You should be able to verify all is well with a cooking thermometer.