Can a cast-iron casserole be used for steaming? I just got my first cast-iron cookware -- a 2.5qt casserole (similar to the Le Creuset ones, but a fair bit cheaper). I understand that cast-iron cookware has some caveats regarding cleaning and storage, and I've heard conflicting advice about water-heavy cooking with them because of the potential for rust buildup.

I'm planning on steaming some potatoes for a recipe this weekend, and while I could use a cooking tray and aluminum foil, I'd like to see how it turns out with a cast-iron casserole; would this potentially damage or cause rust on the cookware?
Enameled cast iron pots, like the one you have, can be treated as if they were steel for purposes of steaming.  You don't need to worry about rust unless the enamel is chipped or cracked. It'll take a bit longer to heat up, but it'll also retain heat better -- so keep in mind that it won't stop steaming until 15 minutes or so after you take it off heat.

If you had a non-enameled cast iron pot, I'd say that you shouldn't steam in it until you had a chance to build up a good seasoning. But that's not an issue for you.