I was wrong, it appears that the Servlet Spec does dictate the behaviour of accessing JSP resources from the WEB-INF directory. Specifically: (taken from the Servlet 2.4 Spec SRV.9.5)
However, the contents of the WEB-INF directory are visible to servlet code using the getResource and getResource-AsStream method calls on the ServletContext, and may be exposed using the RequestDispatcher calls.
I have been against placing JSPs in the WEB-INF directory. Not because it didn't work, but because no-one could show where it was defined in the Spec. If it is not specified officially, then it is a vendor feature and should not be used.
Now that I have a quote I apologise and admit my mistake