I was porting an existing Spring application to GAE/J and was having an issue with JSTL not rendering correctly. It was just dumping the code to the browser.
I finally tracked down the issue to the web.xml that is generated by GAE/J. This issue seems to be that with GAE/J you need to be using the Servlets 2.4 and JSP 2.0 specs for JSTL to function properlyl
You can change your code from:
Edit -- Another possible fix can be found here.