TrendLabs has a nice article on the botnet business model. It features an illustration showing the relationships between different botnets including CUTWAIL, BREDO, KOOBFACE, ZEUS, WALEDEC, and others.

The level of cooperation and coordination is stunning. If you are not monitoring for and blocking botnet activity in your organization, you are exposing your organization to serious risks. If you are seeing no botnet activity in your organization, you are not using the right tools.