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.

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.

Follow up question via comment below...

The WorkflowRule and WorkflowTask appear in the package folder structure as follows. They basically use the /workflows folder and the .workflow file extension. You can read more about them in the Metadata API developers guide here.

P.S. If your interested in the above code it is here.