To connect to Microsoft SQL server on from another machine on Server 2008, there is now an additional step. Windows Server 2008 is the first OS with the included firewall feature already in present in Windows XP and Windows Vista.
You will need to configure a new rule for the Firewall to allow external c to your SQL server.
(If this is SQL server 2005 or greater, you are still going to need to configure your network protocols with the SQL Server Configuration Manager)
Unless your planning on turning off the firewall (not something that I would recommend), then you should add a new Inbound rule to allow this type of traffic.
From the server manager, select "Go to Firewall"
You will then scroll down to the "Getting Started" section, and look for the "View and Create Firewall Rules" section of the application.
At this point, you should think about contacting your infrastructure team if this is for a corporate server, but if this is a simple installation, and you feel confident in doing so, click "Inbound Rules". The on the right hand side, select "New Rule". You will then create a new rule to open Port 1433.
The "New Rule" feature is a wizard, and is really simple to use. Once you've complete your new firewall rule, then test connect from another machine. After a few tries, I was able to reduce the level of access available quite a bit by tweaking the rule itself.
If your looking for more information on Windows Server 2008, SQL Server and developing a cohesive stragegy for managing the firewall, then I suggest checking out this post:
SQL Server and the Windows Server 2008 Firewall