Whenever I enable SMP in VirtualBox 3.0.0 the vboxdrv module ends up crashing and freezing up VirtualBox. I installed a fresh copy of Windows XP in VirtualBox with 4 CPU's. VT-x is enabled and its running on a Q6600 quad core.
Host is Ubuntu 9.04 and I installed the Ubuntu x64 deb package from the download page.
I managed to fully install Windows XP after a couple of tries (VirtualBox kept freezing up requiring a full reboot to start it again because the vboxdrv module was screwing up). Upon shutting down the host it said "Unable to unload kernel module vboxdrv" or something along those lines. After switching back to 1 CPU for the XP virtual machine VBox worked fine, no lockups. I noticed that when installing Windows everything was running very slow.
I also experienced the same on a Vista guest which I already had installed. If I changed it to 4 CPUs it locked up after a while, when I changed it back to 1 CPU it ran fine again.
I am running a custom compiled 2.6.30 kernel. I will try a stock Ubuntu kernel when I get a change.