Problem with disabled VT
Some Core Duo era ThinkPad models ship with Intel CPUs that implement VT-x for hardware-assisted virtualization, but the capability was left disabled by the BIOS. The only known solution is to flash a newer version of the affected ThinkPad's BIOS.
See this LKML thread for more information.
For some reason, VT-x is disabled by the BIOS on some ThinkPad models. Before you attempt a firmware update, you should check this article to confirm that your BIOS version has VT-x as a configurable option.
Please note that the T60, T60p, R60, R60i, R60e, and the G50 are not affected by this problem.
Available BIOS updates
In response to the missing and also disabled VT-x option in the BIOS, Lenovo has released several BIOS updates for the affected ThinkPads to fix the problem.
As of version 2.18 and above, the Z61t, Z61e, Z61m, and the Z61p has VT-x as a configurable option in the BIOS. The X60 and the X60s' BIOS has been also updated to fix the same issue, but it is only available in version 2.06 and above. The X60 Tablet was also fixed, albeit slightly earlier as of version 1.04 and above.