Eating food from a slightly rusting cast iron dutch oven...is it safe?
You're going to die horribly from cooking in a rusted pan!  

Just kidding! A little iron in your food isn't going to hurt you, and can actually help prevent anemia.  To quote On Food and Cooking (pg 790): "Excess iron is readily eliminated from the body, and most people can actually benefit from additional dietary iron."  

Now, to back this up further:

You won't get much iron out of the pan unless you cook something acidic in it.  Rust is insoluble in water without acid present, and in order to become soluble you have to convert to iron nitrate, sulfate, or chloride, according to the solubility table.  Nothing you cook in there is likely to cause the necessary reactions to render it highly soluble.

I found a source quoting specific numbers for iron from cast iron cookware, if you are concerned.  The gist is that most cooking in cast iron added from 1 to 5 mg of iron to the food, with the highest numbers coming from acidic foods with tomatoes.  Applesauce was the highest, good for about 7 mg.  

This level of iron intake is quite safe and healthy. To give you a point of comparison, the FDA suggests iron intake of 8 mg/day for men, and 18 mg/day for women, and 80% of the world may be iron deficient.  Iron toxicity occurs at about 45 mg/day.  So, as I initially stated, you're perfectly safe cooking in your cast iron, and are probably helping your health rather than hurting it!