4/5/2006: This issue has been resolved. Microsoft support gave us a way to disable the new ISA SP2 feature and IIS compression is working again.
3/16/2006: Microsoft had us install Service Pack 2 to our ISA firewall a few weeks ago to try to correct one of the other problems mentioned on this blog concerning the firewall. It turns out that SP2 has a side effect in that web site compression no longer works like it used to. (Technical details: Specifically ISA no longer honors the SendAcceptEncodingHeader that used to allow it to pass through compression done via the internet server. ISA SP2 has it's own compression, but it's buggy...turning it on crashed the server under a heavy load.)As a result the site takes longer to load in your browser than it used to. Microsoft will try to get us an answer tomorrow about how to solve this problem.