Biker wave etiquette.

[HughRussell]

Like Cal said above, in the UK it's mostly a nod, can't remember being waved at, and usually only riders on motorbikes aknowledge each other.
There are a hell of a lot of scooters buzzing around and they don't seem to have time for anyone else.
Using a nod is safer and just as easy to see

[MattyC]

A quick raised hand to any other biker. Honestly, if I took the time to tell whether a cruiser speeding by me in the opposite direction was a HD or any other Japanese cruiser they'd either blow by me by the time I figured out whether I should wave or not or I'd be plastered across the trunk of the stopped car in front of me. If they wave back, fine. If they dont' see me or choose to ignore me, fine.

[yamahonkawazuki]

i dont make preferences on whom to wave to, (like i saw a lot of on the first page.) because there are assholes on both sides of the line . so instead of picking who to wave to i just wave at everyone. including cyclists. i rode a 150 mi charity ride last summer, and was waved at many times. i either nod, or stick left hand out flat, or occasionally w/a thumbs up, if i see a cop, i tap on the helmet, and  move hand (flat down 2 or three times., but to each his/her own.