Outlook Anywhere / Outlook Autodiscovery on Windows 2008 still has some problems.
Read this most excellent post that has all the details.
Long story shorts: Modify the hosts file, remove the IPv6 localhost (::1) and then add hosts entries for your server. I would recommend against disabling IPv6 on the Exchange server, as this is probably not a recommended or supported configuration.
The root cause is that Outlook 2007 can’t contact a DC/Domain Controller using RPC over HTTP/Outlook Anywhere when used on Windows Server 2008.
Also note that NTLM Authentification for Outlook 2007/Outlook Anywhere is broken on Windows Server 2008.