I ran into a little gotcha the other day. Appengine was refusing to compile and process my .jspx files. The solution was horribly simple.
The JSPX format is an xmlized (that is not a word, I know) version of JSP. Check out this question at stackoverflow if you would like to find out about JSPX.
Back to the issue at hand: Appengine precompiles all JSP’s at deploy time. The current version (1.3.4) of Appengine does not let the Jasper compiler kick in if no .jsp files are found in the webapp. However, if you add a dummy.jsp file to the webapp (say in WEB-INF), the Jasper compiler will actually be started and it will pick up .jspx files.