Despite the fact that I have told my dad that this can scare the heck out of a cyclist, he still wants to let them know he's there. Just a couple short little taps. If it's a long "angry" honk, I still wave. I like to think it's pretty hard to feel hostile toward someone who is being friendly.

Although I've had people going the other way honk at me, usually in inclement weather. I wave, hoping they're a fellow cyclist saying "keep it up!"