I am receiving an error message from your test servers where the error code is 2147219093 and the description is in regards to an invalid character.  However, even when I hard-code the HTTP request string (rather than build it dynamically) with the information and send it I am receiving an error code from the USPS test servers.  More interesting, when I copy the endpoint text from the logs in Salesforce I am able to get a response from the test servers when making the request manually using a browser.  The relevant portion of my Salesforce log is listed below.  I get a status code of 200 but it is then followed by an error bad character reference?  Has anyone worked with the USPS web tools that can offer suggestions?

10:09:42.083 (83092000)|SYSTEM_METHOD_ENTRY|[38]|System.Http.send(APEX_OBJECT)

10:09:42.083 (83192000)|CALLOUT_REQUEST|[38]|System.HttpRequest[Endpoint=http://testing.shippingapis.com/ShippingAPITest.dll?API=Verify&XML=<AddressValidateRequest USERID="801CL"><Address ID="0"><Address1></Address1><Address2>6406 Ivy Lane</Address2><City>Greenbelt</City><State>MD</State><Zip5></Zip5><Zip4></Zip4></Address></AddressValidateRequest>, Method=GET]

10:09:42.327 (327010000)|CALLOUT_RESPONSE|[38]|System.HttpResponse[Status=OK, StatusCode=200]

10:09:42.327 (327083000)|SYSTEM_METHOD_EXIT|[38]|System.Http.send(APEX_OBJECT)

10:09:42.327 (327137000)|SYSTEM_METHOD_ENTRY|[39]|System.HttpResponse.getBody()

10:09:42.327 (327212000)|SYSTEM_METHOD_EXIT|[39]|System.HttpResponse.getBody()

10:09:42.327 (327250000)|SYSTEM_METHOD_ENTRY|[39]|System.debug(ANY)

10:09:42.327 (327264000)|USER_DEBUG|[39]|DEBUG|USPS http request: <?xml version="1.0"?><Error><Number>-2147219093</Number><Source>;SOLServerTest.Verify_Respond</Source><Description>Error Loading XML:  A name contained an invalid character.

</Description><HelpFile></HelpFile><HelpContext></HelpContext></Error>