From what I understand about lightning, the material your bike is made of won't make a difference in your chances of being hit. Lightning strikes with millions of volts, and thousands of amps, the 5-6 foot gap from your head to the ground is nothing for that kind of power to traverse. A little metal in your frame isn't going to attract the lightning. Height would be my main concern in an electrical storm. Being the tallest object in the area is not a good situation, nor is standing near a tall object because of the charge that travels through the ground.

"Rubber tires won't help in a lightning strike. The only reason a car is good protection is that the occupants are surrounded by a partial Faraday cage, and that only works if the car has a metal body."

Exactly. Many people believe the rubber in the tires protects the occupants of a car. In reality, if lightning strikes a car, the electricity just travels around the outside of the body to the ground. Again, the few inches of rubber insulation from the tires is negligible when dealing with the magnitude of electricity in a lightning strike.