I was having problems allowing some of my co-workers connect to the instance of SQL Server on my machine. Of course I was aware of the ‘Allow remote connections to this server’ option so I checked that first. It was already on, so I checked out Google to see what I could find.
After fiddling about with firewalls and ports, it turns out that I didn’t have the TCP/IP protocol enabled within the SQL Server Configuration Manager tool. This meant network based communication with any databases on my SQL Server instance was impossible.
As soon as I enabled this protocol and restarted the instance (via SQL Server Services section above) incoming connections were possible. Of course your problem may be caused by something else so check out Daniel Walzenbach’s article on MSDN: How to enable remote connections in SQL server 2008? It helped me a lot.