You can now download all of the latest RTM editions of SQL Server 2012 if you have a currently active MSDN Subscription.
http://msdn.microsoft.com/subscriptions
You can read more about the new features in SQL Server 2012 here:
http://www.microsoft.com/sqlserver/en/us/get-sql-server/try-it.aspx
SQL Server 2012 Express Edition
For those of you looking to download the Express edition, you can do so from this URL: http://www.microsoft.com/sqlserver/en/us/editions/2012-editions/express.aspx