I'm having the exact same problem.
Just started with GWT+Eclipse+App Engine today, so it's quite possible
I'm doing something wrong.
Anyway, I suspected that the code that generates the panels wasn't
being hit and placed a breakpoint there. And hitting the debug (F11)
didn't trigger any breakpoint, which leads me to believe that part of
the code is not running, and that's why the panels are not displayed.
But then again, if I hit the "Compile/Browse" button I can see
everything properly.
Thanks for the help.