ISA in SBS - yes, it's secure: Information on SP2
How off the press! ISA SP2 update has been released. This update replaces the previous hotfix for these issues:
This update addresses the following HTTP issues for ISA Server 2004 Standard and Enterprise Editions with Service Pack 2 (SP2):
• KB 915045: Error 502 "The HTTP request includes a non-supported Header" when accessing certain web servers. This occurs when accessing certain Web servers that return headers that are incompatible with each other.
• KB 915421: Errors 11001 or 400 when accessing certain web servers. This is caused by a misinterpretation of spaces in headers provided by ISA Server, and results in a corrupted URL and failure to load the Web page.
• KB 915422: Event ID 23004 when accessing web sites that respond with compressed content. Some Web servers always return compressed content, which is denied by ISA Server when it did not request compressed content.
• KB 916573: Error 500 (Internal Server Error. Not implemented (-2147467263)) when trying to download zip attachments from an Outlook Web Access server. The header returned by Outlook Web Access causes ISA Server to deny the response.
• KB 917134: Grayed out checkbox “Enable caching of content received through the BITS service”
Supported Operating Systems: Windows Server 2003
• ISA Server 2004 Standard Edition with Service Pack 2
You can download it here or it's available in WSUS.