VirtualBox can't operate in VMX root mode. Please disable the KVM kernel extension, recompile your kernel and reboot. VBox status code: -4011 (VERR_VMX_IN_VMX_ROOT_MODE). Even after the modules were removed (rmmod), the boot up process didn't run to completion. My default runlevel is 5 - but X wasn't started. It was because the VGA card emulated by Qemu is very different from the one emulated VirtualBox. I reckoned I will have to install the 'VirtualBox guest additions' to have the proper vga driver. That also choked with an error.
Please install the build and header files for your current Linux Kernel The current Kernel version is 2.6.Z5-14.fc9.i686 Problems were found which would prevent the Guest Additions from installing. Please correct these problems and try again. After installing the kernel-devel package I managed to install the add-ins but my xserver still refused to start. So I just deleted the xorg.conf file and hoped the system would get by with the default - it did. And finally the graphical login screen appeared - with an atrociously high resolution (fixed through System->Preferences). I have yet to suceeded in enabling the 'seamless mode'. Apparently the xorg version shipped with Fedora 9 isn't fully supported. Now to try an upgrade to Fedora 10. One aspect in which VirtualBox scores over Qemu is speed it appears to be slightly faster. The boot time is 39 seconds whereas Qemu takes 44 seconds.