The Form Based Authentication page for OWA, can be modified on the ISA 2006 (or TMG) server. Fellow Exchange 2007 MCM Kay Sellenrode has made a cool tool to modify the FBA page.
I have created a tool that a lot of ISA/TMG users will like.
It is a Simple tool to edit the Forms Based Authentication page and make it different than what’s default.
Read more at Key’s blog and get the tool here.