Had been fighting a problem with a new SharePoint installation where I had everything set to use "Integrated Windows Authentication" (aka Challenge/Response Authentication Protocol [CRAP]) which should pass your login credentials in the background, but users were still getting prompted for login.

After fighting it for a week and wondering what the deal was and getting nowhere, the network guys finally came through and added the machine to DNS. It had not been and I was hitting the SharePoint server by it's IP address. As soon as the server was added to DNS (And the DNS info was replicated out), the Integrated Authentication began working perfectly.

So, if this happens to you, make sure that the machine is in DNS before you work too hard to solve the problem.