Simple WebService call results in 'function toString() { [native code] }' message

I have a web service and an ASPX page (both shown below) that are attempting to work together to populate a div on the form. I'm trying to avoid partial postback, so I've included the MS js file, but not a script manager. I was able to make this model work in Visual Studio 2005, but it doesn't in VS2008 with 3.5 installed. My TransformXML function works perfectly -- when I debug, I can see that the 'ret' variable contains valid HTML. However, my JavaScript success handler doesn't appear to work correctly. Insteading of getting HTML, I get: function toString() { [native code] }

 Anyone know what's happening here, because I'm stumped. Help is appreciated.

My Web Service:

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[ScriptService]
public class TestService : System.Web.Serv ...