Would Redhat or Novell or any of the others allow me to load Linux on a PC and add it to a Microsoft network that's running ICS, without being a Linux expert? That's what would get me to try it.
Yes. I am running Fedora Core, which is Redhat's consumer product, on my laptop. I have moved it to several networks, including the one at work. It picks up the connection with no problems at all, and I can see all the computers on the network as well as the Internet.
As long as your ICS system doesn't require some sort of client software running on the linux box (just looks like a router to the Internet), there isn't any reason that you couldn't. In fact, using the SAMBA features, you can have 2 way file and print sharing between the Windows and Linux systems.
Mark