Running the launchpad/builder standalone jar from the trunk correctly returns an WWW-Authenticate header on an OPTIONS request with no credentials:
$ curl -D - -X OPTIONS http://localhost:8080/
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Basic realm="Jackrabbit Webdav Server"
But if I replace the org.apache.sling.servlets.resolver 2.1.0 bundle that's in there with the latest snapshot (revision 1302994) that header is missing: it gets removed by the response.reset() call in DefaultErrorHandlerServlet.sendIntro(...), which makes it impossible to connect with WebDAV.
That response.reset() call was not present in 2.1.0.