"The time spent creating a mobile virus could be used to create a windows virus that has a much higher payoff rate."

I'm not convinced. That argument might work for Linux and Mac, but I don't think it sticks here. The number of smart-phones is accelerating fast, and phones are on 24x7. PCs are often switched off. If you infect a phone and make it broadcast spam at 4am the chances are that it will be awake to do so and its owner will be asleep.