As we all know, if the ISA Server 2004 is not part of your Domain, you have to configure a RADIUS server to pass the credentials over to the Domain. But in ISA 2006, there is a new option introduced called LDAP Authentication. Using LDAP Authentication, ISA Server can pass the credentials over to the Domain Controller without a RADIUS Server. You need to make sure LDAP (389) and LDAP using GC (3268) ports are open. That's it!!!!
I don't know why people don't want to try new technology or options if it is available. Many people are still recommending RADIUS server with ISA Server 2006. Anyway, the bottom line is a RADIUS server is not a "requirement" if you are using ISA Server 2006 even if the ISA Server is not part of the domain. At least try to learn some new technologies and take advantage of it instead of just using the familiar old technologies.