Intellisense in SSMS 2012 not working for 2008 server
Try patching your server. I attempted to recreate your issue with SSMS 2012 (11.0.3000) by  connecting to four servers: one SQL Server 2008 (10.0.1600) and three SQL Server 2008 SP 1 (10.0.2531). As you can see the only difference between the four servers is one server does not have Service Pack 1  installed. On all of the servers with service pack 1 installed intellisence worked without issue. However, intellisence did not work on the server without service pack 1 installed.
You can download the service pack here.
http://www.microsoft.com/en-us/download/details.aspx?id=20302