If you are having problems deploying an application, or if the WAR archive generated by the task doesn't behave as expected, it may be that you need more advanced funtionality than the task can provide. It may also be that you have found a bug... In either case, help can be found on the Java Web Start pages:
In order for Web Start applications to be granted access to a user's machine, they need to be signed. The Ant Web Start Task can do this for you, but this requires you to set up a certificate. These links might help with understanding Java certificate creation and usage:
For help with deploying the WAR archive generated by the task: