A metal pan will not crack simply from cold liquid. If you heat it up to a very high temperature and submerge it in cold water, that's not such a good idea. But deglazing is just a tiny amount of liquid.
Now, aluminum is another story - if it's hot enough and you pour cold water (or cold anything) on it, it can warp, even with a relatively small amount of liquid. And for Teflon and other "coated" cookware, you can ruin the coating that way. But heavier steel or cast iron - no way.
If you're really concerned, just keep the deglazing liquid at room temperature. Deglazing is usually done with vinegar and you keep that at room temperature anyway, right? I deglaze my CI all the time with cider vinegar and have never experienced even the tiniest crack in 10 years. I'm pretty sure it's safe. It's great, actually; CI deglazes very easily, you just need a small splash of vinegar.
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!
Best Answer
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.