OT: MS IIS 6 virtual server configuration

Really nothing to do with WebObjects.... this is just the most  
appropriate of the lists to which I subscribe for me to ask this  
question.

Somebody I know has written a .net-based fancy content management  
system for a website that is currently hosted by an ISP with an all- 
UNIX hosting farm.  This is not a problem; we can get the DNS A  
record for the one FQDN for the webserver changed, to leave mail etc  
unaffected. All we need to do is ask for this to point it at this  
someone's designated webserver.

Except I can't see the required content there.  I can run a minimal  
HTTP 1.1 protocol exchange to any other IP address using telnet and  
get back the content I expect.  He's running MS IIS 6.  He insists  
that its all there and ought to be working, just as soon as I get the  
DNS A record changed.

Is anybody still reading that's familiar enough with IIS virtual  
server configuration (ie, in apachespeak, virtual host configuration)  
to suggest why he's insisting that the DNS change has to be made for  
this to work?  It's possible that he doesn't know how HTTP 1.1 works;  
it's possible he thinks there's some quirk to IIS so that the machine  
has to be able to resolve the hosted FQDN as itself, and it's even  
possible that there actually is some quirk to IIS so that the machine  
has to be able to resolve the hosted FQDN as itself.

-- Patrick