Well it took them long enough,, but there is an ISA 2006 certification exam.
http://blogs.msdn.com/trika/archive/2007/11/22/get-certified-on-isa-server-2006.aspx
Brad, I did find some books for ISA 2006, Namely ISA Server 2006 Unleashed.
http://www.informit.com/store/product.aspx?isbn=0672329190