Simply, the Sharepoint intranet site that is automatically installed with SBS 2008 is bound to port 5555 in IIS so to get to it you can browse to server-name:5555 and then authenticate using domain credentials.
I have added another binding on port 80 using a host header so intranet.localdomain.local (and added the required record in the DNS).
This works fine from any computer on the domain, you can go to the nicer address and authenticate no problem. However, when you browse to the port 80 binding on the local server it reaches it but fails to authenticate.
I cannot figure out why and it is really annoying. Not essential to fix but it would be nice. Any ideas?