Workflow Rules have their own component type, as do Workflow Tasks, try this...

    <?xml version="1.0" encoding="UTF-8"?>
    <Package xmlns="http://soap.sforce.com/2006/04/metadata">
      <types>
        <members>Custom_Object__c.MyRule</members>
        <name>WorkflowRule</name>
      </types>
      <version>26.0</version>
    </Package>

You may have to do this for WorkflowTask's as well.