I thought this was pretty funny, I ride a sport bike myself and still got a kick out of this.

Top 10 Reasons Sportbikers Don't Wave:
10. They have not been riding long enough to know they're supposed to.
9. They're going too fast to have time enough to register the movement and
respond.
8. You weren't wearing bright enough gear.
7. If they stick their arm out going that fast they'll rip it out of the socket.
6. They're too occupied with trying to get rid of their chicken strips.
5. They look way too cool with both hands on the bars or they don't want to
unbalance themselves while standing on the tank.
4. Their skin tight-kevlar-ballistic-nylon-kangaroo-leather suits prevent any
position other than fetal.
3. Raising an arm allows bugs into the armholes of their tank tops.
2. It's too hard to do one-handed stoppies.
1. They were too busy slipping their flip-flop back on.

Top Ten Reasons Why BMW Riders Don't Wave Back:
10. New Aerostich suit too stiff to raise arm.
9. Removing a hand from the bars is considered "bad form."
8. Your bike isn't weird enough looking to justify acknowledgement.
7. Too sore from an 800-mile day on a stock "comfort" seat.
6. Too busy programming the GPS, monitoring radar, listening to ipod, XM, or talking on the cell phone.
5. He's an Iron Butt rider and you're not!.
4. Wires from Gerbings is too short.

3. You're not riding the "right kind" of BMW.
2. You haven't been properly introduced.
1. Afraid it will be misinterpreted as a friendly gesture.