This has been discussed in a few places - ISA 2006 is perhaps not going to offer enough to SBS customers, but if you want to have a look, then this is what you need:

Download ISA Server 2006 Release Candidate Trial Software
Test drive the new features and functionality by downloading the free trial of ISA Server 2006 Release Candidate

Internet Security and Acceleration (ISA) Server 2006 Enterprise Edition Release Candidate
http://www.microsoft.com/downloads/details.aspx?FamilyID=a7fd89a0-2f8f-4b71-8a37-efc05724c136&DisplayLang=en

Internet Security and Acceleration (ISA) Server 2006 Standard Edition Release Candidate
http://www.microsoft.com/downloads/details.aspx?FamilyID=7b040fed-64b0-4de9-9d99-bbcfa5779fa2&DisplayLang=en

ttfn

David