Hi Kevin:
sorry for delay in reply, moving back and forth between countries a lot just now.
A typical example of what I mean is at MarisaMartin.com. This is *not* one of the client sites we have problems with - it's a site I have under control myself (it's my wife's

It's hosted on a virtualised IP, and trying to set up the '...hostnames and addresses under server manager and point the www entry back to your ISP hosted website, which solves the problem of local users not being able to access the company website...' simply WILL NOT WORK until I can find a way of sending the HTTP1.1 host_header as well. I've tried! This must be simple, but as far as I can see this is templated and I don't understand the system well enough to change it.
To replicate the issue:
1. get the IP address from a lookup.
2. ping it, fine
3. Browse to the web page at the IP address found and enjoy reading the default page.
JR