I think rain does change the POI. I think the only way to know for sure is to have a skilled shooter that everyone trusts, to shoot say a 308 at 500 yards in the pouring rain, while a skilled videographer captures video to see if any rain drops are actually struck... whether it be by the bullet or it's bow shockwave.
But for now, I would like to present some of my own evidence.
I shoot in every condition Ohio will throw at me, every day. I take one shot as it forces me to understand the conditions around me. I do find a great difference in pouring rain.
An example of me shooting in rainy, windy conditions, that is one shot in pouring rain. My camera doesn't pick-up rain all that well.
http://www.youtube.com/watch?v=RlX3I8PoRP4
I do this every day and I usually wait for the worst conditions. A bit of rain does not effect me in the slightest.
This topic came up on another forum and I made a video today of me shooting in this morning's freezing rain. The load I am using I used all winter, so the 32ºF was not affecting the powder burn. I've used this load from 10ºF to 60ºF. The bore is fouled, and I expect the rifle to shoot sub-moa precise.
First, I aimed with my usual POA and the round hit a raindrop at close to the 50 yard mark, then hit about 12 inches low of the 350 yard target.
The second shot I aimed 12 inches higher, hit 2 raindrops, one at about the 175 yard mark and the other at the 300 yard mark; then hit the target.
http://www.youtube.com/watch?v=dFyQLS2KXBc
I took the rifle back out after the rain subsided to take another shot with my usual 350 yard POA and hit less than MOA from the hit taken earlier in the day.
http://www.youtube.com/watch?v=Up-Mq4tbpnM
I used to tell people that rain had no effect, until I stared experiencing the effect. Now I am convinced rain does have a profound effect on POI, and I would really like someone to set out to either confirm a POI shift due to raindrop strikes, or dispel it.
If any site on the net is able to take on this experiment, it is this site.