Currently I am working on new App Engine application.
Unfortunately it seems that my application uses to lot of resources even for relatively small user number. So
The Eclipse plugin for the Google App Engine provides a local App Engine environment in which you can run your program in debug or profile mode. Perhaps this could provide the information you need?
Appstats: RPC Instrumentation for Google App Engine. It is being developed by Guido himself.
alt text http://img130.imageshack.us/img130/526/timelinea.png
EDIT: I believe AppStats will not work with Java, but there are some tips here.