Also, rather strangely, SFMM still operates two trains on most rides in the rain. Most parks only run a few rides in the rain, and those that do only run one train. SFGAdv can still run El Toro in the rain, but only runs one train. CP shuts down almost all coasters, and the few that remain open run one train (except Millennium Force).
One important not is that, if it rains too hard, or low attendance results from the rain, they may close early. From my understanding, SFGAdv actually does this relatively often. I have even heard that SFMM did this back in 2009, but I am not sure about that. For this reason, it is best not to plan a trip to an SF park on a day with heavy precipitation. However, if you go on a day with light rain, you are probably safe from closure, but the rain may still be enough to scare away some GP.
It rained during ERT at WCB 2011, and I believe everything except Superman stayed open.