This is a quick fix I ran across trying to fix Ettercap. Ettercap comes installed by default on Backtrack5 R2. By default, it also tends not to work and crashes after you launch the Host Scan.
This is the source of the solution http://www.backtrack-linux.org/forums/showthread.php?t=40562 to give credit.
First you’ll need to remove the current version of Ettercap by typing apt-get remove ettercap
Next download these builds of ettercap.
For BT5 32bit go to: https://launchpad.net/~timothy-redae…+build/1758139
For BT5 64bit go to: https://launchpad.net/~timothy-redae…+build/1758138
Download the 3 built .deb files and install them using dpkg -i ettercap-common_0.7.3-1.4ubuntu1drizzt1_i386.deb ettercap-gtk_0.7.3-1.4ubuntu1drizzt1_i386.deb ettercap_0.7.3-1.4ubuntu1drizzt1_i386.deb
Ettercap should now be available in the Backtrack menu under Applications > Internet > ettercap
This will prevent ettercap from crashing after the host scan.