Even though NCCAR was rescheduled, I figured I'd throw my 0.02 in anyway.
1. Autocrossing in the rain is fun. Really!
2. Working course in the rain sucks. Really.
3. Bring an extra pair of shoes and socks, possibly an extra change of clothes. Leave them in your car if you can, or someplace dry. Then you'll have dry versions of each to wear for your drive home.
4a. If you buy rain gear, spend the money on high quality rain gear. The cheap Walmart stuff doesn't breathe well, and gets saturated more quickly. So, you'll be hot, and you'll still be wet.

I bought Patagonia stuff at Great Outdoor Provision and it's fantastic.
4b. Even with good quality rain gear, if you're standing in a downpour all day it'll probably soak through eventually. Sit in your car when you can, grovel for space under a canopy, etc.
5. If it's not standing water, and you've got some amount of tread depth on your tires, you can go a lot faster than you think. I did an Evo school in Darlington SC a few weekends ago, and it was a downpour all day, but the site drained well. It's amazing the amount of grip that was available. That's not just me talking, either, David Whitener taught me that. (He's a national champion level driver, and IMO one of the best FWD guys in the country).
6. The balance of your car will probably change. If you've got a loose setup for the dry, it'll probably be pretty evil in the wet.
Power down coming off of corners is an exercise in patience, but is a great teachable moment. It'll show you what it means to be smooth on throttle input instead of using it like an on/off switch.
7. Buy one of these things for putting your stuff in:
http://www.amazon.com/Highland-1041700- ... detailpageNot my idea originally, Dick Rasmussen discovered these. They're intended for rooftop or trailer hitch mounted cargo carrying. It's very waterproof. Just be gentle with it and don't drag it around on the ground with stuff loaded in it, that'll scrape off the waterproofing and then it's just a bag.