Congrats on your skillet! Cast iron is fun.
Different oils have different smoke points -- that's where the oil burns and (dug) smokes, as well as goes through chemical changes that taste pretty bad.
http://www.goodeatsfanpage.com/CollectedInfo/OilSmokePoints.htm
This page has a list of various oils' smoke points. I like peanut oil, and just picked up a bottle of good stuff from a local asian grocery for cheaper than I expected, but it can be really... peanutty. If you want something more neutral, I have been happy with a peanut oil mixture I got at Target, which is peanut oil and soybean (I think) and is a pretty decent hybrid. It was significantly cheaper than peanut oil, but didn't taste soybean-ey. Whenever I can, though, I use butter 'cuz I loves me the milkfat.
A couple of other things about cast iron -- you may know these, but just in case... Don't put it in the dishwasher, EVER, or use harsh detergent -- you lose all your 'seasoning' and the pan will probably rust in no time. Cooking acidic foods like limes in the pan will also damage the seasoning -- you can get away with a LITTLE lemon or tomato for just a little time, but more than that and you'll probably be advised to use another metal. And beware of rapid temperature changes. Iron is actually rather brittle depending on how it was produced, so try to avoid beating the heck out of the skillets.
Also, a big one -- you can't look at a pan and tell if there's an internal imperfection to the casting, but rapid temperature changes can reveal it dramatically. A friend of a friend told me of a nightmare scenario -- while searing meat in the broiler, the butter actually flashed and caught fire. No big deal, just a little greasefire, but he reacted without thinking and pulled the skillet (thankfully with hot-pads on) out of the oven and dropped it into the sink. The sink was full of cool dishwater. Besides the questionable sense of mixing greasefire and water, his skillet actually cracked -- a fault in the metal parted when it started to cool and about a third of the pan literally broke off.
Cast iron, as others mention, holds heat well. Tall cast iron pots are heavy and more fragile, but rock for deep-frying because they can keep the oil from cooling down when you add the ingredients. A skillet trick my grandmother taught me about making cornbread -- she puts the skillet in the oven while it preheats. Before adding the corn batter, she throws in a tablespoon or so of butter and lets it melt and bake in the pan until it starts to brown, then she pours her batter over that. It gives her cornbread a buttery, crispy bottom that's tasty. I haven't quite gotten the knack of that.
