I have a Core 2 Duo E6600 processor which supports VT-x according to this list:
However VirtualBox (the Oracle version) does not seem to recognize that the processor supports VT-x. I tried to install 64-bit Natty in a VM and the installer indicated that the processor architecture was i686, not x86_64, and terminated. Incidentally the host OS is 64-bit. Also while other VMs run, Oracle VM Manager indicates that VT-x is not being used or detected.
It's my understanding that a processor that supports VT-x and whose OS is 64-bit will allow the installation of a 64-bit VM.
The logs seem to indicate that VM Manager can't detect virtualization on the processor:
though that's not exactly VT-x.
VMX - Virtual Machine Technology = 0 (1)