I would like to point out an issue I faced when trying to dns spoof with ettercap (NG-0.7.3) on Backtrack 5 GNOME 64 bit.
the error message I get on ettercap when spoofing dns is
"Dissector "dns" not supported (etter.conf line 70)"
after spending almost the whole day googling and trying figure out why Im get this error I lucky found a post where someone mentioned ettercap dns spoofing is not compatible with 64 bit operating systems.
So i downloaded BT 5 32 bit GNOME and i managed to DNS spoof without getting that error message.