Connection to local SQL Server 2012 can be established from SSMS 2008 but not from SSMS 2012
I saw an item on Pinal Dave's blog recently that may answer your question.  He suggests adding the line 
127.0.0.1 localhost
to your hosts file

The full article can be found here

I do note, however, the situation he refers to is a machine that is not part of a domain - if your machine is, then this may not solve it