Bad news - Microsoft is pushing 64-bit (as they should), but one of their newest enterprise products does not yet support it. Team Foundation Server 2008 only supports 32-bit. They have said that 64-bit support will be in the next version of TFS, "Rosario".
At least they support 64-bit SQL and SSAS, but definitely not MOSS 64-bit. We have deployed 64-bit MOSS 2007 and it turns out that it doesn't even support 64-bit SharePoint when SharePoint is running on a different server from TFS, it doesn't even recognize that MOSS is installed: