Isn't rain water non-conductive because there's no minerals in it? Unless you live in a highly poluted area where the rain can pickup crap on the way down.
You'd think so, but in reality, no, it always seems to collect just enough dirt, pollen, whatever from the air in the clouds or on the way down. Certainly, I wouldn't count on it.
Along a simular vein, I've seen TX's let the smoke out after being taken out of an air conditioned car and switched on. The TX was cold, the air was hot and humid, and water condensed inside the TX and the smoke got out. So, when you are in any weather that is "condensing", be sure the electronics have temperature stabalized enough that there isn't condensation inside before turning anything on.
On the other hand, a lot of our gear is more water (and oil and gunk) resitant than you might think. All you really need to do is wrap the RX in a plastic baggie, keep water out of the TX, and you're good to go. I've flown in rain a few times, never had a problem with it, and I haven't done anything special. (though I did usually stand under the shelter at the field while flying).