SQL Server 2012 RC0 is now available (click here to download it). For information about upgrading MDS from SQL Server 2008 R2 to SQL Server 2012 RC0, see this topic.
Note: SQL Server 2012 RTM is now available, and RC0 prerelease trial download should no longer be used. 2012 RTM evaluation is available here

The following new features are available in Master Data Services (MDS) in this release.

Use Excel to Manage Master Data
You can now manage your master data in the Master Data Services Add-in for Microsoft Excel. You can use this add-in to load a filtered set of data from your Master Data Services database, work with the data in Excel, and then publish the data back to the database. If you are an administrator, you can also use the add-in to create new entities and attributes.

For troubleshooting help, see this article.

Match Data before Loading
Before adding more data to MDS, you can now confirm that you aren’t adding duplicate records. The MDS Add-in for Excel uses SQL Server Data Quality Services (DQS) to compare two sources of data: the data from MDS and the data from another system or spreadsheet. DQS provides suggestions for updating your data, along with the percent of confidence that the changes are correct.

Load Data into MDS More Easily
Loading data into MDS has become easier. You can now load all members and attribute values for an entity at one time. Previously you had to load members and attributes in separate batches. 

Redesigned and Higher-Performance Web User Interface
The Explorer and Integration Management functional areas of the Master Data Manager web application have been updated with a new SilverLight® look and feel. You can now add and delete members more quickly, as well as move members in a hierarchy more easily.

Installation is Part of SQL Server
You can now install Master Data Services while you’re installing SQL Server instead of using a separate installer, as was required in the previous release. You can install Master Data Services by using the SQL Server Installation Wizard or a command prompt.

Release Notes
Review the full set of SQL Server 2012 RC0 release notes.

In the release notes, there is a list of APIs that are not intended to be used directly from your code. For a list of these APIs, see MDS Internal APIs (XPS document).