Q:

Can I use facelets in a Google App Engine app? 

I would like to do some more learning of facelets.   Now that Java is a supported Google app engine language, is there anything in the facelets implementation that would prevent use on App Engine? 
Edit: This page at google now has a list of various frameworks and their status in regards to App Engine. 

A:

The 1.2 version of JSF was not working - because by default it would try to spawn threads.   That's not allowed in GAE. 
This is fixed in the latest version of JSF 1.2 (build 13), as well as the 2.0 Beta 2 release - and you can download the latest versions from here:  http://javaserverfaces.java.net/
You need to add the a context parameter to your web.xml:
com.sun.faces.enableMultiThreadedStartup which defaults to true.   This
   option must be set to false when deploying to GAE.