Here in the states the JEEP community also does the wave thing.

I've read on the Net that there is even a secret motorcycle language (particularly among Harley riders) whereby leaving a helmet in front of your bike when parked up has significance as does the way that you wave or the tapping of your helmet (eg 'there's a cop ahead').

AA (Automobile Association) patrol men used to salute members sporting an AA badge on the front of their cars.
If the patrol man didn't salute, the driver knew there was a policeman ahead so slowed down.
Incidentally, the AA men then rode motorcycle combinations - vans came later:

http://www.theaa.com/motoring_advice/aa-heritage-vehicles/bsa-m20-and-m21.html

http://www.oldclassiccar.co.uk/aa-motorcycle.htm

http://www.timesonline.co.uk/tol/archive/article4114540.ece?slideshowPopup=true&articleId=4114540&sectionName=PhotoGalleries

Now gone full-circle (scroll down to the bottom):

http://www.twistngo.com/news-twg.htm