After I figured out how to do this indirectly, I found the actual Webmethods REST users guide.  It is a more complete way to implement RESTful services. This only works in version 8.2 however.

I had a rest service call coming into the Integration Server with an HTTP Post request to http://:5555/invoke/TestForKev.esla.pub/event with a type of application/xml in UTF-8.
 

1. So I wrote a flow service in TestForKev/esla/pub called event.  
2. I created an input parameter of contentStream with an object type.
3. I called pub.io:streamToString to convert the contentStream to a string
4. I called pub.xml:xmlStringToXMLNode to create the XML node.
5. Then I called pub.xml:xmlNodeToDocument to finally convert to a webmethods document.

6. URL Parameters

   
   If you need to take a rest service input that uses URL parameters, you can still get to them.  Say you have a url like http://:5555/invoke/TestForKev.esla.pub/event_change/98765 where 98765 is the event_id.  
   
   
   Just write the service as above, but add in a call to pub.flow:getTransportInfo.  The requestURL returned will contain the service name and the parameters at the end of the url, so you can parse them out.