I was thinking about this too. That "pay us 300 dollars in bitcoins to get a key to your data" virus could be like that.
If the payment -> key sent part was automated then money would just build up in the bitcoin account over time.
If the payment -> key sent part was automated then money would just build up in the bitcoin account over time.
Of course, without the creativity to come up with new C&C mechanisms, the pattern of migration and purchases might eventually become predictable enough to stop the botnet.