Can I eat food cooked in cast iron to leach iron into the food? Is it a significant amount?
Can you get leached iron from cast iron? Yes
Is it consistent in the amount that you get? No
Is it dependent on the food in the pan? Yes
How much? It varies hugely dependent on the food. More liquid, acidic foods leach more. How long its in the pan also plays a role. 3 oz of spaghetti sauce could leach as much as 5mg - that's almost 1/3 of the daily recommended 18mg.
So you can cook with cast iron to get more iron, but it will vary wildly on what you cook in it. It will also likely increase/decrease based on how well the pan is or isn't seasoned.
Given rfusca's answer above and Newman's comment, I thought it might be worthwhile to see what medical autorities -- or what passes for such on the internet -- have to say.
"There is real evidence that cooking in cast iron pots increases the amount of iron in the diet. This is usually a very small source of dietary iron."
"Cooking with cast iron pots can add up to 80 percent more iron to your food."
McKinley Health Center says dietary iron can be obtained from:
"A NON-HEME food cooked in an iron pot, such as a cast iron skillet"
Columbia Health gives the most solid information about dietary iron from cast iron cookware, also containing the only reference to an actual study on the topic. It also reinforces rfusca's answer that it depends strongly on the type of food and how it's cooked.
So, the answer appears to be yes, you can get dietary iron from cast iron pots, and it's a significant amount. Not that that gets you out of needing to eat a nutritionally balanced diet.