I nod, but many don't and thats not always because they're ignorant. I don't think they notice other road users. I have a mate who never nods, who I asked if he nodded at other bikers and he said yes always. I thought the was odd as he didnt. After a ride out, and being behind him and passing half a dozen bikes that nodded at us, and that he didn't nod back I did my own little test. I said to him, did you see that bike go past up by the junction.... no, did you see the CBR etc etc .... no. He only see's what he is concentrating on. Bit like car drivers. So he never actually registers in his brain bikes passing. Funnily enough I won't ride in front of him as he nearly hit me once!