ISA in SBS - yes, it's secure
I've added a few more links. These links point to ISA community resources. The trick with accessing non-SBS ISA community is that in the SBS world we sometimes do things a little differently because of the compromises that have been made to allow ISA run on and protect a DC. Just be careful out there and don't forget to use the wizards when appropriate.
The Official Microsoft Community
Microsoft ISA Server Newsgroups
Microsoft TechNet How To's
Microsoft Support for ISA2000
Microsoft Support for ISA2004