Based on the @agarcian’s suggestion above, I googled the error:
restsharp Data at the root level is invalid. Line 1, position 1.
and found this forum: http://groups.google.com/group/restsharp/browse_thread/thread/ff28ddd9cd3dde4b
Basically, I was wrong to assume that client.Execute was going to be able to auto-detect the return content type. It needs to be explicity set:
var request = new RestRequest(Method.GET);
request.OnBeforeDeserialization = resp => { resp.ContentType = "application/json"; };
This could be cited more clearly in RestSharp’s documentation. Hopefully this will help someone else out!