Mmm... nope?
I've always been able to do it with just a restart/Option hold.
Am I supposed to only be able to do it cold? 😀😛
Perhaps I haven't made myself clear. If you boot into OS X first
and then reboot into Windows, it may be OS X that is enabling
VT-x. That's why that method can work when a cold boot
straight into Windows doesn't (on some machines, at least).
Not everyone wants to boot two operating systems to get
something to work in one, you know
🙂
That said, it may be that Windows 7 is smart enough to turn
on VT-x if it is unlocked and hasn't already been turned on.
And finally, not all Macs may behave equally with regard to
VT-x and whether or not it is enabled by the emulated BIOS.
There is much to find out here. If anyone wants to give it a
go, apparently CrystalCPUID can tell you if VT-x is enabled
(although I can't confirm this as I haven't tried it myself):
http://crystalmark.info/software/CrystalCPUID/index-e.html