Passing Variable Into A Facebook Iframe App

Hi All
I’ve created an IFRAME APP for facebook which works fine, but I need to pass a client ID across to the IFRAME so it pulls the right client page every time from my server.
ie facebook user 1 clientID 1
facebook user 2 clientID 2

The client sets up the app as
facebook.com/dialog/pagetab?app_id=***&next=&app_data=clientID

Can this be done or am I going about this all wrong

Steve