This is a big announcement made at WinHEC http://www.microsoft.com/presspass/features/2008/nov08/11-06winserverr2.mspx 

“[…] we continue to invest in scalability. In Windows Server 2008 R2, we have built in support for up to 256 logical processors, which will allow our customers to more fully exploit today’s powerful CPUs, deploying only the features they choose and scaling those solutions to meet their organization’s needs.”

You can watch the recording here http://www.microsoft.com/presspass/events/winhec/default.mspx

[UPDATE] - Go to about 32 minutes; I particularly like the shot of task manager with 256 processors (about 38 minutes in)