ettercap is bugging in Backtrack 5 R1 32bits and 64 bits versions fully updated
ettercap version: NG-0.7.3

In ettercap filter
Code:
if (ip.proto == TCP && tcp.src == 80) {
log(DATA.data, "tmp/log.txt");
}
From the log file, I can see that DATA.data doesn't contain the full TCP/UDP payload but only the TCP/UDP payload at index 434 (0x1b2).

The following picture shows an hexdump of log.txt file that displays the content of DATA.data
http://imageshack.us/photo/my-images/12/datadata.png/

The following picture shows ettercap http response within Wireshark
http://imageshack.us/photo/my-images/254/wireshark.png/

This bug is not present in Backtrack 4 R2