i'm actually considering building something very similar, but using a TSL230R light to frequency converter... sadly, however, i'm in atlanta and rain is rather scarce at the moment... though, the sensor itself is very sensitive and dealing with a number that directly represents a light level is easier to interpret... i am simply polling directly in the sketch loop, which is as fast as the arduino can process... that said, i've really no way to test against lightning at the moment (but i missed a storm last night!!)... maybe i'll just put it in the dark and turn lights on/off quickly... or maybe fire a flash in the room?...
http://www.taosinc.com/product_detail.asp?cateid=2&proid=14
- isaac