I just installed BT5 (32-bit GNOME) and discovered this build did not have the pcaprub extensions installed. So, Metasploit won't do anything with the raw socket modules except give an error. In the interest of saving others time, below is a cut-and-paste from the Metasploit wiki on how to do this (Note that the Metasploit wiki says "# cd /pentest/svn/framework3/external/pcaprub", but that's incorrect for BT5)

# cd /pentest/exploits/framework3/external/pcaprub
# ruby extconf.rb
# make && make install

Hope this helps!