Recently installed SQL Server 2008 on a Windows 2008 box and was happily adminsitering it from the console of the local server. When I fired up SSMS (SQL Server Management Studio) from my development machine, it wouldn't connect.
I turned the firewall off and still couldn't connect.
Turns out TCP/IP isn't turned on by default. Maybe I knew that at one time, but this time it totally slipped my mind.
To turn it on, go into SQL Server Configuration Manager, expand "SQL Server Network Configuration" and change "Disabled" to "Enabled" by right-clicking on "TCP/IP" and suddenly, I could connect from my development machine.