Did you know that Windows Server 2008 installation, by default, enables the Windows firewall to be ON just like Windows Vista.
So there is a possibility that in the previous installations, you have not been using the Windows Firewall and disabled it for your day to day work. When you start working on this new installation of Windows Server 2008, you may see the connectivity issues from remote clients to the SQL Server.
If you see connectivity issues to the SQL Server from remote clients in Windows Server 2008, then refer to this blog by the Microsoft SQL Server Security team. In this excellent post, Shawn Hernan from the SQL Security Team takes you through the steps to configure the Firewall settings on the server for the SQL Server.
For enthusiasts to know more regarding the Windows Firewall settings specific to the SQL Server access, please refer to this document on MSDN.
SE, Microsoft SQL Server