As of today you can download a evaluation version SQL Server 2012. For purchasing SQL Server 2012 you have to wait until April first. For more information check out the official blog post: SQL Server 2012 Released
This is the first SQL Server release I am proud to be part of, the AS team has delivered tons of new features in the BISM, tabular projects, Power View integration, more DAX and of course a new version of the PowerPivot add-in. I think this is great release for BI with Power View now added to the stack as great interactive data exploration, visualization, and presentation experience.
This release also includes the long awaited 64 bits version of the Data mining add-in, which you can download here.
PowerPivot for SQL Server 2012 is now here as well, you can download it here.