During last football season i shot in a few rainy games.
What i did, was take a plastic bag, cut a hole for the flash head, just behind the flash head i took a thick rubber band and wrapped it around as tightly as i could. I then took electrical tape and wrapped the snot out of the hot shoe.
The head actually seems fairly well sealed, i'm not suggesting it IS sealed...just that it's fairly tight. The parts that worried me were the joints and buttons, thats why i kept those covered.
Only once, at the end of a game, did problems occure, the flash would completley discharge on its own. After looking stuff over a little bit i came to the conclusion that it was due to a short in the hot shoe created by water.
I think thats the biggest problem, really.
Just my expereince