Ok, then I'll assume this is the thread for posting our "Windows 8 as host" observations. Hopefully, others may chime in with their own experiences, and we may all learn something useful.
As reference, my host hardware is an early 2011 MacBook Pro laptop with a quad-core "Sandy Bridge" processor, 8 gigs of RAM, and a 500 gigabyte SSD partitioned to run both Windows and OSX via Apple's "Bootcamp" manager software.
My Windows 8 Consumer Preview (64-bit) OS was installed as an upgrade over Windows 7 Ultimate (64-bit), and as a general rule, the overall experience has been problem-free.
I am using VirtualBox 4.1.8 with up-to-date guest additions in all VMs which have them available. All VM's have all available OS updates installed - I've included a list of VM's at the bottom of this post.
I currently have identified the following issues while attempting to run Windows 8 Consumer Preview (64-bit) as a VirtualBox (VB) host system:
1) Windows uses my MacBook Pro's discrete GPU - the ATI 6750M. Previous to Windows 8, I had used the most current ATI driver from Apple's BOOTCAMP setup. Windows 8 update offered a newer driver, labeled in its description as an "Engineering Sample" for this chipset. When using this newer video driver, VB throws a "LibLoad" error at multiple stages during its operation: at startup of VB, when attempting to start any VM, when attempting to edit any part of any VM, when attempting to use any VB command-line-based tool. It may or may not be of interest to know that the updated video driver from Microsoft's Windows Update site is listed as WDDM 1.20, rather than WDDM 1.10. Unfortunately, the LibLoad error dialog fails to list which particular library is not being found.
Reverting the driver from the WDDM 1.20 from Windows Update to the Apple-provided WDDM 1.10 driver allows VB to run without the LibLoad error dialogs or messages.
In either case, however, VB continues to run - the LibLoad error does not cause it to crash or result in a host BSOD. However, the problem with the WDDM 1.20 driver *does* prevent the successful start of any VM's. Reverting to the WDDM 1.10 driver allows my VM's to run as expected.
In using the older driver, presumably, one has to give up the additional features and benefits of the WDDM 1.20 driver on Windows 8, although I'm not conversant with what those features/benefits may be or how much utility is being lost by not using said driver. Windows 8 so far seems to be perfectly happy with the WDDM 1.10 driver.
2) As is otherwise noted on another thread in this forum, use of Microsoft's Hyper V virtualization product results in VirtualBox not being able to start any VM's. I mention this here because, unlike Windows 7, Windows 8 appears to be the first consumer-facing OS from Microsoft that will support Hyper V as an optional install, which means there are likely to be more VB users in the future than there are now who experience problems due to this incompatibility. For me, simply uninstalling Hyper V allowed VirtualBox to start VM's again.
3) Bridged vs. Host-only networking issues: I have two issues here, one related to bridged networking capabilities and one to host-only capabilities.
Bridged: If the bridged networking capability is installed (as is the default) with VirtualBox, my host system will 100% reliably lose its wireless Internet connectivity each time after waking from sleep. Recovering Internet connectivity requires running the Windows 8 "troubleshooter" each time upon wake and allowing it to perform fixes to the wireless adapter with Administrator privileges. Configuring the wireless adapter's "Power Management" tab options to allow/disallow powering off the device to save power make no difference to this behavior. At this point, my work-around has been to install VirtualBox *without* the bridged networking capability, which appears to prevent this unfortunate issue. Luckily for me, I have no real need for bridged networking on any of my VM's although that was the way that I was using them previous to installing the Windows 8 preview.
Host-only: This is simply a minor annoyance, and it's likely not due to anything in VB at all, but to the way Windows treats active network adapters, but the host-only network adapter (which does not have Internet connectivity on my host, of course) is non-configurably displayed with the yellow exclamation-point warning about "limited connectivity" in my system tray. My wireless adapter is functioning correctly and has Internet connectivity, but its icon does not appear in the system tray at all as long as the host-only adapter is present and enabled. On Windows 7, the OS appeared to ignore the host-only adapter when an active, Internet-connected wireless adapter was also present. The current state of affairs results in Windows 8 only displaying an icon for a single adapter in the system tray, and it unfortunately defaults to the host-only adapter, which it dutifully reports as having a "limited" connection with the distracting yellow exclamation icon. Even more bothersome is that this prevents my seeing the somewhat useful signal-strength indicator for the non-limited connection on my wireless adapter. The work-around I've found for this is to "disable" the host-only adapter when not running VB. This allows the wireless adapter's connection status and signal strength to be displayed in the system tray (when not using VB), but requires me to remember to re-enable the host-only adapter each time before running VB.
I mention this problem even though it's probably not VB-related, since it seems possible (likely?) that this behavior will make it into the Windows 8 commercial release, and I am somewhat hopeful that the VB developers could somehow contrive to install a host-only adapter device that contained some kind of internal configuration that would keep it hidden from the Windows 8 user-interface altogether. Otherwise, I forsee a number of future postings on this forum asking for assistance with how to make Windows ignore the host-only adapter and its "limited connectivity" overlay when they only want to see the wireless adapter in their system trays.
Other than these issues, I have been able to use VirtualBox without incident on my 2011 MacBook Pro running the Windows 8 Consumer Preview (64-bit) with the following VMs:
Windows ME (don't ask...)
Windows XP Professional (Version 2002) SP3
Windows 7 Ultimate (32-bit) SP1
Ubuntu Linux (11.04 Natty Narwhal)
Fedora Linux (15)
gparted live (dedicated partition management OS, boots & runs from ISO)
OSX (Leopard) - not stable - even under Windows 7 or OSX hosts
BSD Unix (no X, console mode only)