Virtualbox and KVM

At the moment I'm using virtualbox on my Ubuntu 8.10 laptop for running windows (very occasionally) and some SLC test machines for work. Oh and a few other Linux Distros too. Anyway, much as I like the point n clickiness of virtualbox I thought I'd see what the 'built in' KVM could do, esp as libvirt looks like a nice abstraction layer.

However on starting virtualbox machines I get the error:

VirtualBox can't operate in VMX root mode. Please disable the KVM kernel extension, recompile your kernel and reboot (VERR_VMX_IN_VMX_ROOT_MODE).

well, thats not strictly necessary - a simple sudo rmmod kvm_intel kvm and relaunching the virtualbox guest.