The paper The use of Business Rules with Workflow Systems by Anthony Rowe, Susie Stephens, and Yike Guo points out that there are two classes business workflow systems, process-centric and data-centric, and that business rules should be able to express rules that operate in both worlds.
The authors point out that this is not an "either/or" situation, but rather leveraging both workflow models in the design and execution of workflows is the way to go:
"By combining the two different workflow models, business rules can be undertaken at both the task level for automating different decisions and at the data level for implementing filters over the data. Business rules can also be used to define operational features of a workflow, such as what to do when a specific task fails."