i have noticed here in arkansas that patting the top of your head indicates that the police are up ahead
i'm guessing this patting gesture means roof lights but i do know it works

When I started riding many years ago, you would only wave at someone if on they rode a bike like yours. Harley guys waved at Harleys. Jap bikes at other imports. I wouldn''t get a wave back from a Harley rider while on my Kaw 650. I believe times have changed and for the most part, anybody will wave at anybody. What you are riding doesn't seem to be as important. The wave is a sign of the brotherhood we all share while riding.
I take patting the head as "heads up". It can mean LEOs, accident, or just use caution ahead.

Here are a couple sites I found that may shed more light on the subject.
http://www.viewfromthecloud.com/2006/08/secret-motorcycle-hand-greetings.html /> http://www.ama-cycle.org/roadride/groupRideSignals.asp />
Johnny

I was told that it is a friendly wave to all bikers, but if your hand is downward it is a sign for downed biker, or if riding in a group there is crap in the road.

I personally wave to anyone and everyone who is on the road on two wheels.   Harley, metric, sport bike, dirt bike..I don't care what they are riding, as long as they are out enjoying the open road on two wheels.

Sometimes I even forget I'm not on my bike...I have hit my had on the truck door several times trying to wave to a passing rider 

It's the attitude!!!

There are all kinds of riders out there on all kinds of bikes.  Some just try to be cool when they wave, others like myself are usually pretty relaxed when we ride so it's kind of a drop of the hand with a couple of fingers out.  and then there are those who wave like it kid going past on a roller coaster.  I guess they are just really excited to be acknowledged by another rider.  Doesn't really matter to me too much how they wave, I usually wave back - even if I do have a little chuckle at their expense.

I took the MSF course in May of '07 my instructor taught the class the 'wave'. I liked the idea and so waved at everything (hand low two fingers extended), A week after my course I went to bike week in Laconia, NH. BOY did my arm get tired!! I did see in the beginning not many of the Harley riders returned the gesture (I ride a crotch rocket). Since then things have changed, now 99% of the people wave back but if I'm at a bike week or when I was on the dragon... I didn't wave half as much unless someone initiated and I wasn't on a curve demanding my focus.

We often use the circling of the index finger pointing down as a reference for upcoming police. As so not to let them see you patting yourself on the head!!!

I've noticed a lot of the so called yuppie riders don't have a clue as to what any of the gestures mean. When riders do, I figure they've been around the lifestyle at least for a while. Not ALL of us were born into it the way I was.

we have a 03 yamaha 1100 V star classic, most of the people we ride with are HD riders we wave at anyone on 2wheels so as long as your knees are in the wind its all goodsometimes i think goldwing riders dont wave because there is no button for it ha ha ha ha ha

Guess I'm gonna have to learn all of these. All they taught me in class was left, right and stop!

here are some of the hand jestures from a passing bike, i've learned from my time rideing. granted these may not be true for every where but they all make sense to me:
the friendly hand out with two fingers down is for keeping the rubber side down. i usually do two fingers down with thumb for the saying -- keep the rubber side down and shiny side up!

if you pass some one and they have there palm down like patting the ground that usually means slow down. speed trap ahead.

and as mentioned earlier patting the head usually means they passed a cop car so beware.

ride hard and keep the shiny side up!!!

just like tootall says

He forgot the middle finger tho which is a favorite of mine with the drivers we all put up with.

Ok, to help out with some of the questions on different motorcycle riders hand signals, I have created a blog with all of the hand signals, at least the safety and informational ones.

Motorcycle Hand Signals

I try to way to every motorcycle rider I pass. I have no particular wave, just depends on the mood I'm in and how lazy I am that day. Sometimes I just kind of roll my hand of the left side of the grip, other times I give a full wave.

Only one no one mentioned that we use around here is pinkie finger and index finger extended up from top of helmet(think rocker gesture) means deer/cow/elk...you get the idea!Dont wanna hit them!

Great subject, as a relative newbie, I've been wondering about it.
I usually get the top two fingers or open hand, mid or low, mostly harley riders, but also a lot of metric cruisers and some crotch rockets. I usually try to return a wave unless I'm braking or in a turn, then I just go for the head nod). As for what someone's riding, if they're on a motorcycle of any kind, I'm happy to wave or return a wave (unless the in a turn or braking as before), however I have mixed feelings about scooters and mopeds and such, I have mixed feeling there, with the exception of little kids on bicycles, some times they get a kick out of seeing a motorcycle roll by.