From its corporate blog the Microsoft Windows Server Division leaks some details about ongoing Windows Server 2003 SP2 beta program and reveals:
Service Pack 2 improves the performance under high APIC access rate for Windows Server 2003 running as a multiprocessor guest operating system under Windows Virtualization.
Read the whole post at source.
The improvement depends on how Microsoft is now handling interrupts with Service Pack 2: they will be put on hold to collect as much as possible before asking hardware to serve them.
This way hypervisor will not have to do translations for every interrupt.
While Virtual Server 2005 will not receive any benefit from this change, performances improvement will appear when Windows Server 2003 SP2 will be used in the Windows Server Virtualization (WSV) as guest OS, or in any other virtualization platform supporting virtual SMP (including VMware products).