As a part of the broad initiative across Microsoft to support 64 bit versions across many of its product lines, the next release of OCS will support 64-bit operating systems only. This decision will help meet customer demand and is a natural progression of the product that aligns with the same approach taken by the Exchange team (with Exchange 2007) and the SharePoint team (with SharePoint 2007) to support 64 bit operating systems only.

Read more at source: http://communicationsserverteam.com/archive/2008/0...6.aspx