Interesting. I've run into the issue with single NIC/single IP, but NT auth works with named pipes. However since it's disabled with most servers (sockets only), it was blind luck we figured that out.

It started when we changed the service ccount. I read Chad Miller's article and suspected rights on the service account (domain user, normal) in creating the SPN, but resetting to an admin account didn't fix it, for some reason, only going back to the old service account, which was shared with other services on other boxes, worked.

Then a few months later it cleared up. No indication why.