(repost from XLNX forum)

Has anyone succesfully managed to program the eFuses? On the Virtex-6
in particular but any device would be good...

There seems to be many restrictions and I am unclear if I am violating
one of them.  If you have managed to program eFuse please respond
with:

Device
Impact Version
JTAG scan chain

Anything else that might help.

Thanks, Tom

Hi Tom!
I have programmed the eFuse AES key of the Spartan-6 LX100 on our
board using Impact 13.3 and a Platform Cable USB II. You have to
connect the RFUSE pin to ground via the specified bias resistor and
the JTAG cable can not be at more than 1.5 MHz, but if that is
fulfilled it seems to work. At least configuration with an
encrypted .bit-file passes. We use 3.3V VCCAUX, I can't remember if
that is a factor...
Cheers!
/Lars
P.S. Remove the obvious from my address if you want to email me
directly. D.S.