I have just posted a script at http://www.bruningsystems.com/rtlsio.p
that allows one to snoop incoming/outgoing packets on a Realtek NIC.
The script is very easy to change for any other GLD-based NIC (see
the comment at the beginning of the script to determine what needs
to be changed.)
To run the script, save it and then:
# dtrace -q -C -s ./rtlsio.p
Let me know what you think.