this is now the 4th part of the Reverse Proxy Saga going on with this network (Part 1, Part 2 and Part 3). today I have implemented an ISA 2006 server sitting in front of the web servers. it actually seems to be working quite well, plus I have the advantage of everything being compressed outbound, if the client supports it.
The problem with Squid, which was originally running the site, was that outbound compression was not supported. Also, I got the impression that Squid though of Windows as a second class citizen. not everything was supported on Squid on Win32, which was a pity.
I am currently using it as a Virtual machine on my main workstation, but I hope to move to a full, non Virtualized build soon. If your interested, ISA info is here, and the VHD image (30 day trial) is here.