Thank you for your reply.
I wish the page to be available to BOTH logged in users and anonymous visitors.
Facebook [Fan] Pages act like this i.e they are visible to search engines and can be seen by anonymous visitors.
The problem with setting 'RequireLogin=true' is that the page is then hidden and shows only a logon box.
The reason I want to obtain the uid for logged on visitors is that the page will then display ADDITIONAL functionality for any logged on users.
I can already do that to a limited extent by using <fb:if-is-app-user> but this does not help me with user specific information,
What I am basically saying is that I don't REQUIRE users to be logged in but if they are I want to know who they are.