Running Windows 7 XP Mode under Virtualbox

Does anybody know if it is possible to try out "XP Mode" in Windows 7, if Windows 7 is running inside of Virtualbox? The processor I'm running includes the VT-x capability and it is enabled in the BIOS. Theoretically VT-x allows for nesting (ie. running Xen inside of an other instance of Xen), however I couldn't get the guest Windows 7 OS to detect the availability of VT-x.

Does Virtualbox support this? What do I need to configure? Alternatively, does VMWare support this?

Update: thank you to everybody who responded. After doing further research and experimentation myself, I found that this currently isn't possible, although theoretically it would be (my original motivation was to play around with XP Mode without physically rebooting the machine – I guess I'll have to byte the bullet and do a full install / reboot physically). Further reference:

From VMWare community (emphasis added):

It is only possible to run nested VMs when the outer VM uses hardware virtualization (Intel's VT-x or AMD's AMD-V) and the inner VM uses the classical BT (binary translation) monitor. You may run any 32-bit or 64-bit operating system as the outer guest. You may only run 32-bit operating systems as the inner guest. These configurations are entirely unsupported.

Virtual PC 2008 uses the equivalent of "hardware virtualization" not "binary translation", and as such, it is incompatible with the scenario described.

From Invisible Labs (emphasis added):

We can now virtualize complex hypervisors, like e.g. Virtual PC 2007 or Virtual Box with SVM turned on (BTW, we can also run VMWare Workstation, but that doesn't count, as on AMD processors it doesn't make use of SVM instructions). We also have a prototype code that allows to run nested hypervisors on VT-x but that code requires a bit of more polishing (oh, didn’t you know that our NBP also supports VT-x these days?).

The conclusion is that it is technically possible, but no product implements it yet. I will award the bounty anyways.