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:
<web-app xmlns="http://java.sun.com/xml/ns/javaee" version="2.5">
<web-app xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee" version="2.4">
Edit -- Another possible fix can be found here.