1) Install VMware-esx-supptools-3.0.0-1.i386.rpm from the ESX 3 cd.
2) You can then run “/usr/lib/vmware/support/esxnet-support man” and learn how to use the tool, but …
3) To get a trace from the portgroup, you’ll need to run:
/usr/lib/vmware/support/esxnet-support trace -z -p Network0
Network0 of course will need to change to your portgroup name.
Another thing you can do is create a portgroup and put it in promiscuous mode. Then, run tcpdump in that guest that’s in that portgroup.