The real answer is it depends on what you think what responsibilities exactly ESB or Workflow Orchestration should have. dragonwriter is of the reasonable opinion that ESBs focus should be a message plane. If your of my opinion, ESBs generally include some of the features found in workflow. devonkim is also right in that ESBs in general let the business data/process bleed into the rest of the stack, where as with workflow, its completely agnostic to the business data/process.

The most accurate answer would be from comparing various ESB products to Workflow Orchestration products, as every vendor/product will have slightly different opinions on where their responsibilities are.