The issue was found on Mac 10.7.3 64bit with jre cobundle build 339. The ant fx lib is from:
http://jfx.us.oracle.com/hudson/view/2.2/job/2.2/label=macosx-universal-14/lastSuccessfulBuild/artifact/artifacts/sdk/lib/ant-javafx.jar

Open http://sqeweb.us.oracle.com/net/sqenfs-1/export1/comp/jsn/users/larry/bugreport/dtfailure/html/testDocumentbase_JNLP.html in browser, the webstart application can not loaded. The page is using dtjava.js generated with latest ant-javafx.jar.

Same scenario with working dtjava.js:
http://sqeweb.us.oracle.com/net/sqenfs-1/export1/comp/jsn/users/larry/fxcases/fxfeaturesTest/html/testDocumentbase_JNLP.html