Tony,
Actually, you are very incorrect. Driving in the rain does NOT clean the undercarriage of your car. It has the opposite effect.
Rain does NOT wash greases and oils off of your car. I accidentally spilled a little transmission fluid on my driveway a few weeks ago when I changed the wifes alternator, and even after 5 inches of rain (no kidding - wet spring here in MA) there is still tranny fluid on the driveway. So on point 2, you are not correct. If you want to try to prove me wrong, put a small amount of any type of oil you want on the hood of your car, right before a rain storm, and go out the next day to see what is there. You'll see.
The dishsoap in and of itself is not a big problem, but the contaminents that it removes goes right down the sewer and into the local water supply. We've already covered that.
