rain-with air kit
A good question and one I asked myself when looking at air kits. With the thunder air kit you can buy a "rain mask" that is baisically a plastic/nylon streach ring that goes around the filter that is permiable enoungh to let it breathe without sucking in water. They sell for about $12.00. I bought one and keep it in my saddlebags for cheap insurance. If you are trailering your bike you should probubly just cover the filter with duct tape to protect it. I've only driven in rain a few times since adding my thunder air kit and have had no problems at all with it. Actually the only time I had a problem in the rain was with the standard issue paper air filter on my 800A which became soaked and the bike could not run until it was dried out. Most rain showers won't harm a K&N filter as it is impregnated with oil and repels water, but obviously you would not want to put the water hose or pressure washer on one.
Hope this helps!