The OCS Team released a note saying that the next version will support x64 OS.
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.
For the next release of OCS, Microsoft will support deployments on 64 bit operating systems – Windows Server 2003 x64 (R2) and Windows Server 2008 x64.
Read more at source: http://communicationsserverteam.com/archive/2008/08/29/246.aspx