2008 licensing
"The transition to multicore is happening even faster than the move to 64-bit, and Windows Server 2008 is multicore-ready. Microsoft is also licensing by socket, not cores, on a chip," Laing said. " (reported elsewhere)
If true, this would be the most compelling reason to upgrade from Windows Server 2003 Standard to Windows Server 2008 Standard Edition. 2003 Std only supports four cores - if 2008 Std supports two quad core chips, that would allow 2X scaling above that of 2003 Std.