The misconception that metal roofing somehow attracts lightning likely stems from the fact that metal is a highly conductive material and from a basic misunderstanding of the circumstances involved with lightning strikes.
To briefly answer the questions, no. Metal roofs do not attract lightning, and are not more frequently struck than any other roofing material.
What happens during a lightning strike?
First off, it's important not to confuse "attraction" with "conduction". Yes, metal is an electrical conductor which means it allows the flow of and electrical charge in one or more directions. This doesn't mean that metal "attracts" lightning any more than any other material.
Lightning is a rapid discharge of electricity in the atmosphere that happens intra-cloud (IC), cloud-to-cloud (CC), or cloud-to-ground. The location of a cloud-to-ground lightning strike is not determined by material, but by geography, topography, and storm movement. As water droplets inside of a storm constantly move up and down inside the storm cloud, they inherently separate positive and negative charges from one another. Once enough negative charges collect at the base of the storm it will induce positive charges on the ground below. As a result a potential difference is formed in the air gap between the cloud and the ground. The voltage will then continue to increase until the air gap between the cloud and the ground become electrically conductive and discharges a stepped leader. This is the first step to a lightning strike, and is invisible to the naked eye. Once the stepped leader nears the ground, a streamer (collection of positive charges) fires upward usually from the tallest object in the charged area, to connect with the stepped leader. Once this connection is made it allows electrons to flow from the cloud to the ground and positive charges to flow from the ground back up to the cloud. This flow of charge creates a flash of blue-white light that we recognize as a visible lightning strike.
Now that we understand a little more about the conditions that are necessary to produce lightning, you can see why the idea that metal roofs attract lightning is flawed. A tall building with a metal roof may be prone to more frequent strikes, however, this would be because of its height relative to the surrounding topography, not because it has a metal roof. The roof could be constructed of any material other than metal and it would not increase or decrease the frequency in which the building was struck by lightning.
Ok, so what happens if a metal roof IS struck by lightning?
Because metal is an electrical conductor it does not offer much resistance to an electrical charge. This is the idea of a lightning rod. When a lightning rod is struck it grounds the charge to the earth's surface because it is conductive. A similar effect can occur when a metal roof is struck by lightning. Instead of resisting the charge, it would allow it to pass down toward the edges of the structure. If metal framing was involved it would produce the lightning rod effect of passing the charge to the ground. This is why some people choose to ground their metal roof systems.
Secondly, steel is not combustible. So if your metal roof was to get struck by lightning it wouldn't catch on fire (that's not to say that other materials close by wouldn't).
In conclusion, metal roofs are not impervious to damage from lightning strikes but they do not attract lightning, are not struck any more frequently than other roofing materials, and may even provide more protection than other materials if a strike does occur.