When you install WS08 as a Guest OS then the WS08 media includes the earlier version of Hyper-V Integration Services, so network and other hardware devices will not work. If you launch the Integration Services CD it will tell you to go to Windows Update to upgrade the Integration Components (Catch 22 I guess).
The workaround is to 'Insert Integration Services Setup Disk' start Device Manager (devmgmt.msc) find the failing device, select 'Update Driver Software' and 'Browse my computer for driver software' - the path is then e.g. D:\support\amd64.
Second, I noticed the following limitations in multiprocessor support in Hyper-V (In the Release Notes)
For the guest operating system, install one of the following:
- Windows Server 2008 with Hyper-V, with a maximum of 4 virtual processors. No other release of Windows Server 2008 is supported with this release of Hyper-V.
- The Windows Server 2003 operating system, with a maximum of 1 virtual processor for 64-bit and 2 virtual processors for 32-bit.
If this is also true for the RTM version (Haven't checked yet) then this product is less useful than expected for 32 bit deployment of WS03 and practically useless for 64 bit deployments of WS03 (Even though Exchange 2007 SP1 supports Windows Server 2008, many companies won't have approval/testing for this product and/or their system management, backup etc. products ready for WS08).