What do you know, it happens in Singapore too! Here we have a public transport system so well-developed you can't sit at a bus-stop for 5 minutes without seeing at least 10-15 buses pass you by. They wave too.
I've noticed something else especially peculiar here. In Singapore, we drive on the left side of the road. The bus-drivers sit on the right-hand side of the bus. When 2 buses get close enough (e.g., when waiting to turn right at a cross-junction while it's still red), the bus driver on the right is usually obliged to open the passenger swing doors on the left and make small talk with the bus driver on the left (who simply shouts through the window).
There seems to be some unofficial rule that all bus drivers must learn a Chinese dialect, and not just any dialect, but a common one. I am ashamed to say that though I am Chinese I have not taken the tiem to learn my native dialect, and thus I have no inkling what they talk about during these little exchanges. But I'll try to get a friend to look into it, now that secret bus driver waves seem to be everywhere else as well.
And it's not just the lollipop men (sadly, these are slowly being phased out by traffic lights that hardly break down), the bus-conductors, station control, even some of the passengers are in on it as well! I'm serious about the passengers! Some of them get up at a stop, chat a little with the driver, then get off 5 stops later - without paying a cent. I didn't suspect a thing until the passenger waved to other bus drivers as well, and he kept sneaking glances towards the back of the bus. Freaky people.
Somebody should start a master node for conspiracies like these.