The beta version, released in November last year, has been downloaded by some 7.5 million people, more than three times the number who used the beta of Office 2007. The new version is the first to be available in both 64-bit and 32-bit versions, though the 64-bit version is primarily to allow developers access to a 64-bit version so that they can update extensions and add-ins (to pave the way for users to migrate to 64-bit in the future), and the 32-bit version is still recommended for most uses. Other features include a new ribbon user interface for Outlook, and greater support for Windows 7 features like Jump Lists.
Software Assurance customers will be able to download the new products starting April 27th, with Volume Licensing purchases possible on May 1st, and an official launch event will be on May 12th. There's no word yet of when MSDN and Technet subscribers will be able to get hold of the new version, but typically this happens at around the same time as SA availability. Pre-orders for retail copies can be placed immediately.
Source: ars technica