Another snob thread, all right!
I wave if/when I can. But unless I'm going really slow, I usually don't take my hands fully off the bars to do so; just raise/extend my hand. I'll say "hey" or "hi" or something if they're within earshot.
I think it's kind of cool when the motorcyclist "brethren" wave to each other, and it's nice when other cyclists wave back (or initiate the waving) but in the end I don't really care either way. It's not like I come home crying. But I do think the appearance of a friendly attitude is not a bad thing at all, and should be promoted.
Some non-wavers probably are genuine snobs. Some probably believe they can't safely execute a wave (though how they can signal obstacles and turns, yet not be able to wave mystifies me). Some are probably just oblivious. Whatever.