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][1].

![enter image description here][2]

![enter image description here][3]

P.S. If your interested in the above code it is [here][4].


  [1]: http://www.salesforce.com/us/developer/docs/api_meta/Content/meta_workflow.htm
  [2]: https://i.sstatic.net/21rms.png
  [3]: https://i.sstatic.net/iHGc8.png
  [4]: https://github.com/financialforcedev/apex-mdapi