If I am doing anything where I need both hands, I'll give a quick nod. Otherwise, I'll usually lift two fingers off of the left handlebar. _|/
If someone comes up along side me or I come up along someone going close to the same speed I'll say hello. I usually don't start a conversation, but I'm not against it and there have been many times where I've had a nice chat with another rider while pedaling 22mph up the road.