I've heard that pouring a cold liquid into a very hot cast-iron pan can destroy it (because it's more brittle than a steel pan for instance). Is that true or do most of you deglace in cast-iron pans as well?
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.
Cast iron is more brittle, sure, but cast iron is practically indestructible, so that's like saying "diamond is more brittle than jello". You probably could shatter it, if you tried really hard, but there is no way you'd do it just by dumping a little liquid into the pan, while it's at a normal cooking temperature. Even if it was way beyond a normal cooking temp, throwing liquid in the pan would just cause some really energetic steam.
Now, run it up to 1000 degrees or so, then lob it into a bathtub full of icewater, it'd probably shatter.